As usual, it depends.
Comments where? How long are they going to get? How many comments are going be there? Very short comments don't need titles, and you also don't need titles where you aren't likely to get a high number of comments, so you can display them all and you don't need to save space. Facebook doesn't support titles to comments, and there's no need for them.
On the other hand, many news websites support titles for talkbacks, and it's a good idea. They show the talkbacks collapsed, and they're extremely useful if you want to get the general spirit of the comments without having to actually read them all. I can form an impression about a hundred talkbacks in the time it would take me to read three (and that's if I don't have to expand them).
For another example - Livejournal is a blogging platform where it's not uncommon to get thousands of comments, in a possibly extremely deep branching. So titles are very helpful there. The problem is that livejournal only decides to collapse comments after a certain limit is reached, and in most cases it isn't - so people are used to their comments being expanded to begin with, and they rarely use titles. But on websites where visitor know that their comment is going to be collapsed, they use titles in very smart ways.