I can envision what this forum would be like if members had the ability to lock their own threads, and it ain't pretty. While there are a number of people here who I'm sure would not cause a problem in the slightest, I can think of quite a few who'd go to town playing backseat moderator. For instance, say that someone posts something in your thread you don't like, what's to stop people from saying 'if you don't agree with me, I'll lock this thread!' or otherwise just acting obnoxious and power hungry?
Perhaps I'm a bit biased, but I think moderator controls should be left to moderators. I just see this causing more problems for myself and the rest of the staff than it can solve.