I support the idea, for all users to do it.
There's always going to be people to abuse the system, so I also support rules to be in place for warnings being issued if someone abuses the system.
I also think that when people flag a thread, it should re-direct them to another page where they have the option of writing in why they're flagging the thread. This can prevent the accidental slip of a mouse and also be useful to staff as a lot of people would probably take the time to write a few words on why they're reporting a post.
I for one, don't really like having to send in a Support Ticket to report things, although I do if I see something serious. I'd feel like a snitch if my Support Ticket list got too long with reporting stuff. xP