We have too much junk up there as it is. Everything from adult astronaut diapers to floating hammers, to rocket-booster engines.
It's a good idea to reenter large objects like a space station.
Most satelites only have enough fuel for 4-5 years of manuvering (Depending on the satelite). So once that's gone, the satelite may still function...but will be unable to stabilize or correct it's orbit.