The motivations for hiding one's identity are almost all bad (personal attacks, trolling, unpopular opinion) and far outweigh whatever legitimate (if any) reasons there are for a forum alt.
Practically speaking, whichever character on the computer has the most playing hours is most likely the main identity and should be the default forums identity as well.
If someone doesn't have the moral courage to be identified with their post, then it should't be allowed.
Unpopular opinion is a "bad reason"? ...Really?
By the way, a lot of people have "forum alts" for wiki access. If you're using a Steam-connected account, you can't do anything on the wiki...