I view replies to be similar to notes in not needing/requiring titles.
For metadata it can often be useful to have an actual title in the admin UI to identify the post in those administrative views.
What I find even more interesting/useful is having the ability to put in a custom title for automatic POSSE purposes, and the title is often used by many (most? all??) POSSE plugins for just this. In this UX/UI case it can mean some of the content could be displayed multiple times, once in the title and again in the body, which isn't ideal.
I'm certainly in the boat that stripping the title is a theme issue. I'd argue that for the best possible broad usage, Post Kinds should strip it out (using CSS) as a default and allow those who'd prefer to display it on the front end, just untoggle the CSS setting.
If this is the case, then the final consideration is the duplication of content between the title and body within RSS as well.