Ideally, if you have a bit of information that further supports or improves an answer, you should add it to the body of the post. More often than not, folk who simply want to chime in with interesting anecdotes or comments like "+1 That happened to me, too!" add a degree of noise that simply is not what comments were designed for.
Comments are there to ask for clarification or to help improve the post. Supporting information should be contained in the post itself.
If a comment isn't directed at changing the post or causing some other action to be taken, they start to take on the role of miniature chat rooms, and that type of ongoing discussion folks have to dig through to get to the "finished" answer is exactly the type of activity Stack Exchange was designed to avoid.