Why we should use other sites when Bitcointalk Administration can easily prevent it?
It's just in case of emergency, it's still better to have your own evidence of a particular suspicious page than to wait for Administrators to provide evidences. Sometimes it is necessary to do something than to just sit down and wait for things to be solved.
Like what I said, since I don't really know if Admins can have a copy of edited messages /replies, it is much better to use Archive.fo to save a copy. Quoting the message/reply is a good idea too but other members can also edit the content of the particular reply so it means someone else can also be able to change its contents.
Well I think I'm right, after doing some research I found out this reply from theymos that might give you some idea.
Can admins see editing history of a post?
Yes, though it's a bit of a hassle.
There's currently no history of profile fields, though, so you definitely can't rely on your signature or anything like that.