See I don't know about this. I can see where you'd be coming from but at the same time I don't want to promote people using the boards for something they are not.
This place is meant to be for discussion, if you were just going to post something then delete it right away then it shouldn't have been posted in the first place.
I'd rather not have people start creating topics and making posts just to get a simple question answered and not allow further opinions or responses to be made after, that's just not how the system should work IMO.
Now, I could see your case if for instance you made a post and then realized it was a bad idea to post it after the fact, but if that's the case all you have to do is edit it saying that you've changed your mind, then flag it and we can delete it for you.
(Plus then there's the whole thing where what if you deleted your post but it leaves all the responses to it still visible. The topic just becomes a mess of confusion at that point. And if we gave you the ability to delete your post AND the responses to it we'd be giving users too much power and control over someone else's free speech)