I use this site to get training in writing english. I also use it to try to be less shy. Sometimes I write long posts that I delete because they are not comprehensive enough.
I used to do that a lot, although I mainly deleted them because I felt the post I'd written would add little. Which is a bit silly really, given the average quality of posts on gaming forums is... well, no quality whatsoever.
But anyway, yeah, I think it's pretty common to write out full posts only to end up deleting them before hitting submit. You should consider hitting submit instead of deleting them some of the time :)