[Burrytar]

Exp is not a reward if it fails to grant access to something a player actually wants. If empty exp cannot influence the behavior of players, then it cannot influence gameplay. (It could influence the behavior of players if total exp is scarce and empty exp is analogous to hoarding.) What this means for idlers is that they are ultimately brought back to the game by expectations of what they can do in the game -- by the promise that there is a reason not to idle.

So idlers who routinely come back to the game probably fall in two camps:
1) People who want to play this game but have to leave the game to sustain that lifestyle.
2) People who don't want to play this game, but do want to play a different game that they see as accessible only by playing this game.

In the first case, you need a good idea of what the idler thinks of as "this game" in order to know whether idle exp is a good or bad thing. A person who enjoys the game may be miffed that when they come back the exp has pushed them into a different game, or they may be glad that they haven't been pulled back into a different game (such as being drastically lower rank than their friends).

In the second case, idle exp is probably a good thing, so long as it only changes the game significantly for the person who doesn't enjoy the current game. Even if the change turns out to be much worse than they expected, so long as the cost of getting there was insignificant, then the information adds to their ability to choose what to do next: press on, start over, or go elsewhere.