The problem of sub 50 rep users being unable to comment is well documented
The rationale for the 50 rep limit is good: reduce spam
As evidenced by many of the answers to those questions, the problem that this limit is trying to solve is that of spam.
The justification is simple: we don't want the site to be flooded by comments from passers-by. ЯegDwight
The canonical answer to this question is found here. Because it does such a good job of summarizing the points, I'll quote it directly.
However, history and experience have shown that the downsides of allowing everyone to comment are far greater than a few useful comments lost:
There are big problems with spam. Automated filters cannot catch all of it.
Even among the real comments, most would either say "I have the same problem" or "I agree". Such comments do not add any value, and have to be manually removed.
Comments are very painful to moderate. Stack Exchange sites have a process of community moderation (voting, flagging, review queues) that works great for questions and answers, but not so much for comments. Comments cannot be downvoted or closevoted, nor searched (nor do we want that).
Comments are second class citizens on the Stack Exchange network, not designed to hold information for all eternity. They may get cleaned up at any time. Generally, truly important information should be incorporated into an answer anyway (either by posting a new answer, if the information answers the question at least partially, or by editing an existing answer, if the information is a minor complement or clarification of that answer).
Clearly, we need a solution that reduces spam and maintains the site's goal of high quality information. In partial opposition to the quality goal is the goal of effectively on-boarding new users. We want new-users because we believe the average lifetime contributory value of a new-user is non-negative.
The 50 rep limit is not good for new users
The purposes of comments are these:
Request clarification from the author;
Leave constructive criticism that guides the author in improving the post;
Add relevant, but minor or transient information to a post (for example, a link to a related question, or an alert to the author that the question has been updated).
The problems introduced by the 50 rep limit are these:
A new-user is not be able to ask the clarifying question they have on the answer they have found
A new-user cannot add clarifying information which could help a future user
#1 is bad for the user, #2 is bad for the site
"But it's easy to get 50 reputation"
The very common response to this problem is that it is easy to get 50 reputation points. Here's how to do it:
Correcting the grammar of either questions or answers. To do so, click the edit link by the entry. Rephrase poorly expressed needs. For many, English is not their native tongue.
Answer questions. Each time another member upvotes your answer you receive 10 rep.
To simplify Andrew's and other people's recommendations: #1 edit and #2 answer so that you earn the rep you need. It's easy.
However, both solutions fail under scrutiny.
#1 Editing is a limited option and non-trivial
Editing, while an option for native speakers, is not really an option for non-native or poorly educated speakers. Presumably, we'd like to avoid discriminating against users based on English mastery. This leaves only number #2 as an option to those people.
Furthermore, the 25 edits needed to reach the 50 rep limit are not nothing. If it takes only 20 minutes each, that means over 8 hours of editing to reach the limit. It certainly takes me more than 20 minutes on average to edit an answer because I want to make sure the edit balances between substantive, without changing the original meaning of the author. Effective editing is also predicated on prior knowledge which brings me to the problems with option #2.
#2 Question answering has many barriers to entry
It's a question/answer site, how hard can it be?
Stack Overflow is known for having stringent guidelines for what constitutes acceptable answers, deviating from those guidelines can result in reputation hits. To avoid breaking rules, there is significant prior knowledge required about the site. By definition, new-users don't have that prior knowledge. The intro-tutorial only gets them so far (and only up to 10 rep). This dearth of context reduces a new-user's confidence in their ability to participate. Anecdotally: it did for me.
Furthermore, answering a question requires not only information about what constitutes a good answer, but also an answer to the question being asked. New users are disproportionately more likely to be new programmers, therefore their ability to answer questions is much much lower. To demonstrate how I felt as a new user:
I touched programming for the first time about 9-12 months ago. Only in the last 2-3 months has my level of competence reached the degree where I can provide valuable answers. In other words, [answering questions on tags like]
dockerhave not been available to me as options, because until recently I knew nothing.
It gets worse. Not only are new-users nervous about messing up how they answer questions, while having less to provide in terms of an answer, they're also handicapped in terms of their ability to collect information in order to answer the question well. They cannot comment for clarification.
Consider this quote.
To make requests for clarification, or mere responses to other answers, you need to wait until you have the comment privilege.
While there are ways to get around it (see "But I can't write a good answer without more information!" in community wiki), there is no denying that a new user is at a disadvantage in this respect.
In short, it is not easy to get to 50 reputation points. It is much harder to get from 0 to 50 rep than it is to go the next 50 to 100 rep. Just look at a couple of users' reputation graphs; they frequently follow a non-linear trajectory.
This is a problem that can be solved
There are certainly bad solutions to this problem.
There are also workarounds that are not acceptable:
Can I put my comment in an answer?
But there are some off-the-cuff ideas from high rep users that garner support.
I was never a fan of the 50 point threshold. My two cents would be to allow <50-rep users to comment, but if any >1,000-rep user flag their comments, then the 50 point threshold gets put in-place instantly. Just a brainstorm. – LarsTech
The first comment on this answer
The minimum reputation limit for commenting on Stack Overflow is intended to prevent spam and noise. Stack Overflow is constantly attacked by spammers and trolls, so some safeguards are needed. We don't have a similar reputation requirement for questions and answers, because we have moderation tools that help us identify and remove spam and undesirable content coming in via those post types.
The minimum reputation requirement for people to leave comments is a constant source of frustration for new or less active users. People find something factually wrong or in need of correction in an answer, and they don't want to go and have to spam answers to get enough reputation just to comment. Many turn to leaving answers (since they don't have a reputation requirement), which then clutters up the site and wastes our time in cleanup.
Maybe it's time to think about relaxing this, contingent on proper tools being made available to moderate comments.
How do we solve the problem of the 50 reputation comment limit?
To steal from Brad,
[Solving this problem] would both reduce user frustration and clutter from non-answers that come in every day.
A note about my reputation
My low reputation is my credibility on this matter. This is a frustration that I have been dealing with since I signed up for Stack Overflow a little over 1 year ago. What finally provoked my frustration was a scenario where I needed the version number of a program that someone had used in an answer; I couldn't ask them about it. My solution was to download every version of that program and run their code until I could replicate their error to verify the version number. One comment would have circumvented that ordeal. That's untenable.
I'm now above the 50 rep threshold, but I'm unwilling to abandon this issue. I'm concerned that this is the type of problem where people with influence and the capacity to make change are unaffected and therefore unmoved. I hope I have managed to convey the problem, a potential solution, and at the very least manifested a conversation worth having.