Multi-boxing in FFXIV legitimately is not very efficient (two computers, two monitors, two keyboards, two mice, two accounts).

I can't speak for using third party software tools. Don't use them myself.

I think it depends on the game. When there's enough to do for one player or the mechanics don't allow for it easily, it takes away joy trying to multi-box. Where in a game like Aion, some battle mechanics on quite monotonous and it gives way to using triple-boxing.