If you’ve played SWG, you know space and ground are separate. There’s a divide there. When you earn space-xp, it doesn’t help you very much on the ground and vise-versa. In Lotro it’s the same story for creep play and freep play. Nothing you do with your elf feels like it has anything to do with your orc.
When I think about all the different kinds of xp to work on in games, such as crafting xp, entertainer xp, and faction/reputation points, I think there’s a similar feeling of separation. It’s like having a large number of games and mini-games smashed into one big game.
On the other hand you could have a world like EVE. In EVE there is no xp per se, just Isk (Money). Whatever you’re working on, you’re working on Isk. Could you imagine if all the fantasy MMOs and Sci-Fi MMOs out there only had the gathering of cash as the sole form of advancement?
Having a lot of different kinds of xp or different games to play in one MMO makes for a lot of variety. On the one hand, doing vastly different activities to advance different things makes for a lot of variety. Sometimes you can be working on two or three things at once (XP / Cash / Quest rewards) and it feels extremely satisfying. On the other hand, if you get really excited about something like PVP or Space-xp, it feels like that’s the only thing that’s in the game. Everything else is just a diversion with rewards that don’t influence your “real” game.