Wouldn't be very good for UX to force only Latin characters. Not all sounds can be expressed with them. Would you join a forum that forced you to use Chinese characters? How would you even begin to create a username?
I don't think impersonation is a big deal. Only a small amount of impersonators actually succeed in pulling off a scam, and the victim is unlikely to get tricked again. Over time it becomes harder to come up with a username that looks similar enough and to pull off a scam before getting banned.