I dunno. They're the same as any game, entertainment. The only difference is whether or not you want to play alone (SP), with someone (MP), or want to have a variety of people you can meet (MMO).
The only one I'm playing is Vindictus, which plays like Soul Reaver (I've been waiting for so long for someone to make an action MMO that is not point and click) and is free. So, I get free entertainment that is more than stat building and item collection. No different than playing a SP RPG.
Besides, I play like an hour every couple of days. No real loss.