[KaVir]

I recall one mud I used to play where you had two types of rent - the standard one, where you'd pay X gold per day (run out of money and everything vanishes) and a sort of cryogenic sleep where you had to pay a few days worth of rent in advance, but it was a flat fee, and lasted until you next logged on (for people going on vacation and such).

The latter tended to discourage casual play though - if you were on vacation and happened to spend half an hour at an internet cafe, you couldn't log on to say "hi" without reactivating normal rent. Likewise, if your time was limited (exam, work deadline, etc) you had to completely block yourself from the mud until you had more time again.

Because, as others have observed, those players will often come back. Sometimes not for months, or even years, but they know they've got a character waiting for them if they ever feel like dropping by. Sometimes a new feature catches their attention and they start actively playing again - often bringing in new players from whatever other game/s they'd been playing.

If someone's only played for a few minutes or so, I'll eventually wipe them, but if they've invested time into building up their character I see no reason to ever delete them. It's not as if they're taking up much HD space.