Something I haven't said much about is why, despite its negative side effects, I generally like EVE Online's offline skill training model.
The answer is pretty simple: because the negative side effects produced by the online XP collection model are worse.
Here are some of the problems that come to mind when considering an online-XP design:
1. It promotes mindless grinding behavior instead of exploring the gameworld and story behavior. It damages immersion to run into a bunch of people just cranking through some trivial action over and over and over again for XP... and that kind of design actually attracts such people, making the problem even worse. (No, not everyone considers this a problem, but some do, and they're paying to play the game, too.)
2. It allows players to level up quickly. Once they're at max level, unless they like raiding (assuming the game even has raiding) or want to start up a new character (and not everyone does), they're done with the game. They leave, and stop paying a subscription fee.
3. It's rare, but there have been some cases (esp. in South Korea, where it's estimated that something like 30% of the population plays online games) of people dying from neglecting their health to play for three or more days straight.
4. A design that lets people level up as long as they keep playing can't possibly be good for their hygiene....
