I say, it is an okay idea, but definately not on the top of the to-do list. You can usually tell if someone quit, or if they were going to be gone for awhile, they'd be smart to at least say why they are leaving and for how long. (Depends if they come back)
well if someone tries to logon with THAT account and THAT password plokkey, then there should be a popup screen like "your accout has been deleted to innactivity"
It's not that necessary, unless you have friends and you want to talk to them by posting on their profile. But then if you're that eager to talk to them and wait 'til they're on, you should get each other's instant messaging screen names. xD