It's not specified in the rules outright, but it's safe to say it is highly frowned upon. The reasons for this are plentiful, obvious and already stated by yourself. The rules contain warnings against roleplaying in the chatroom, but that's that.
It might not be permanently banned outright, but the user might be temporarily banned, if he/she doesn't listen to the mods. Not that it happens a lot though. People generally don't impersonate others directly. For example, if some guy with the username darktyphlosion posts crappy answers, then you know it's not the real DT. Or if someone pretends to be Rex then you know they're lying because dinosaurs have been extinct for millions of years and nothing can bring them back (except InGen, but there's a trilogy of movies explaining in graphic detail why that's bad).
That said, sometimes there's confusion due to two users having the same IP address due to them being siblings, so to prevent confusion, they're asked to specify in another Meta post.
Of course, if someone's impersonating someone with bad intent, then obviously the person will be warned, and consequently, banned.
If you think someone is pretending to be someone else, you could post on a moderator's wall, and they'll check the IP address to solve the issue. For example, ƒιzz is one mod who is quite active compared to others, so you could ask him nicely to check something out for you.
