At the moment I believe that the ability to reply to and quote locked threads is vested in the "Lock / unlock threads" forum moderator permission.
I agree that it might be useful to have the option of allowing certain usergroups to quote locked threads independently of the lock/unlock permission for moderators.
Naturally, this would require multiquote to be enabled - since "reply" in a locked thread doesn't make sense if you don't have the ability to bypass the thread lock like the moderators with the above mentioned permission do.
I'm not sure I would enable that on my sites - but threads do get locked for many reasons, not only because they are problematic - sometimes it's just because they are old. Quoting an existing post in an old locked thread to start a new discussion is a perfectly reasonable use-case in my opinion.
It's almost like we need another setting for closing threads. "Locked" vs "Locked and nobody can quote posts" ... I would use the former for old threads and the latter for problematic threads.