Yep. I Do this quite a lot in the Cross Validated site. It is the only way to get up votes or the correct answer check. If you take too much time to craft a well thought out answer, somebody will beat you to it and get all those. So in order to get the best of both worlds, I anser fast and then, if needed, improve on it.

>For some reason people are loath to downvote an early answer without competition, even if it's rubbish.

Yeah, that's because downvoting answers cost reputation. If not for that, people would be waaay more willing to downvote

... but that would also bring a practice of downvoting good answer made by others to boost your own