As convenient as this would be, I can really see some people changing the names of threads constantly just to drive people crazy. You shouldn't usually have to rename a thread, you can always make a new one. In sections such as Advertisements where you are only allowed to have one thread at a time, you can easily post on the sticky to have it deleted. And it's always a good idea to check over your thread ti
tle to make sure it is typo-free before you post it.
I do see where you're coming from, and there are occasions when you would like to rename a thread, but I think it may get too out of hand. As Shadwolf pointed out, anyone could drastically change the name of a thread, which would be an annoyance if nothing else.
I think it would be easier and less of a bother to simply check over your titles for typos and create new threads when you absolutely have to.