This question already has an answer here:
Low-rep users can't leave comments. This leads to many users posting their comments as answers.
While the rationale behind this limitation is understandable, seen from the outside, it looks inane and bureaucratic. New users often have useful information to bring to the table. That information is often enough destroyed through deletion, or has to be manually converted to a comment by a moderator.
How about, experimentally for a month or so, removing the limitation, allowing comments from (registered) new users, and countering the threat of garbage comments with
A comment review queue for low-rep users' comments
A lowered flagging threshold for low-rep users' comments (say, 2 flags instead of 3 or whatever is required right now)
After all, features like anonymous feedback show that there's value to be gained from the input of the countless visitors who end up on Stack Overflow through Google, but are never going to become active members of the community. So why not use it?