I always try to reply. Especially when people put effort into their comments. The rewards for comments are not what they once were, but comments aren't necessarily about just making money. Comments can bring on collaborations, followers, and even if not rewards in steem dollars, it's worth taking the time to spend a lot of effort commenting.

Some of my comments I'm sure are longer than some posts people make, but when someone writes something interesting I believe they deserve the same thought in the feedback/comment section.