Lets see this in another perspective, perhaps a more to-core explanation.
Stack Exchange is a Q&A site. It it's core, it's all about asking and answering questions.
The need for comments exists because, among other things, we are human beings and sometimes fail to provide all required information for a question. Even if we think we have provided all information, someone might say that there exists a similar, if not duplicate question already answered.
Comments exists to add or ask for relevant information, without adding an explicit answer.
The current limit of 50 reputation exists as a privilege. As all privileges, they are given as a result of the work an user does within the community and as such represent a certain level of trust. More so, if comments are used to ask information, they represent a certain level of commitment; the user that asks has spent an amount of interest in the question.
Besides the reasons pointed by Mr. Peanut (spam mostly), a new user is less likely to use the comments their intended way (as some users come here thinking this is some sort of forum). Also, the answers given by new users aren't generally good.
If a question needs to be commented in order for it to be solved, chances are that it isn't all that great a question in the first place (as seen, the linked question has one downvote) and, by influence, the answers given wouldn't generally be that great either.
An answer, however, is the core of the site, and as such, it is allowed for even an unregistered, near-anonymous user to post one. There is already a system to deal with non-answers that, not being perfect, it deals with this problem effectively.
There is a slight upside in this system of non-answering/deletion - the (unsolved) question gets bumped, thereby placing it on the front page, where it might raise new interest.