Take this question for example. It's a good question, but there's no definitive answer without going through some of the initial troubleshooting steps I point out in my comments in order to isolate the problem.
We are encouraged only to answer questions in our replies, rather than respond with speculations as to the problem - but in this case, and in many cases, isolating the problem using the comment section is crucial to actually delivering an answer. Should there not be a "reward?"
Because comments are transitory, and should be used to indicate clarification etc. An upvote on a comment is further evidence to the OP that it's a good one that may allow them to amend their question/answer. If that results in an improvement etc, then the vote should be applied to the question/answer (if that's not already been done), not the comment, which should then be removed.
Essentially, comments are post it notes. In the ideal world, they all get binned once they have served their purpose to further the efforts to improve a contribution. Comments can spill over into chat if too many are created, and as such it's just like starring a chat comment - it signifies it adds something, but it isn't a formal part of the q or a.
On extremely busy posts, votes push up the best comments while the rest are hidden from view, so votes on comments aren't in the same rep giving class as actual posts.
