Comments on Stack Exchange have a very clearly defined, yet temporary purpose. They are intended to seek clarification on a post or to help a user improve that post. Comments aren't intended to answer questions. If someone has an answer to post, they should post it as a full answer.
Additionally, comments aren't ranked like answers are, and keeping useful information in comments potentially hides it, since that information isn't as visible as an actual answer.
Since the goal of Stack Exchange is to build a high quality repository of questions and answers, putting the focus on comments would take away from the design goal of focusing on the Q&A; thus, this isn't a feature likely to ever be implemented.
If you see a comment that is valuable, ask the poster to turn it into a full answer, or do so yourself. Then flag the comment as obsolete so that it can be deleted. See the "When should I comment" section of the comments privilege in the help center for more details.