IMO, posting under a different identity being good or bad depends on the reason. If you are entering some contest where only one entry is permitted, and you create a new name for that purpose, then of course, it is bad.

With that said, I own a forum and I have 2 identies there. One is me, Rizzano, listed as the Founder, and the other is a joke name to provide entertainment. Of course, most people think it is me, but it's left unsaid.

It just depends on the motive behind it, I guess.