When you post something on a forum, you don't actually own anything, you share the content you post with the Forum Administrator, Forum Moderators, and other forum members.
You don't see in everyday conversation when the topic runs dry the person who brought it up saying "Off topic, I own this conversation!!" It really makes one look like a fool.
I think the same should apply here, but I say *should* because this isn't like everyday conversation, this is the internet, you don't have that actual face-to-face conversation going on. Instead of facial expressions, you have emoticons, which some people can't or don't use due to visual impairments.
Claiming conversation ownership in real life is ridiculous, so I think claiming thread ownership is also ridiculous. We should let the conversations flow naturally, and so what if a thread gets derailed. There are more tragic things in this world than a derailed thread.
JMHO.