I delete my comments regularly few weeks after posting them, because often my paranoia kicks in, and I feel like I have to at least cover my tracks a bit. I do however, do my best to leave up all comments that answer something or have relevant information.
When I switch accounts to a new one, I delete my full comment history, though. I know this is bad for the continuity and history and everything, but I have not figured out ways to lessen my paranoia. And it does not help, that I somewhat lack in emotions, and do not care much, what people I do not know, think about me or my actions. So my options are, either keep doing this, or stop commenting. So far the first one has won.