It is kind of a pain in the ass for the players to be facing off against Mr. Superdemon or something, and then to have it all come crashing down because he couldn't make the Will Save -4...
Well, to be fair, there are a LOT of spells that have this issue. Especially at this level. Finger of death, disintegrate, etc Heck, Banishment is lower level.....at least this only banishes them for a day. The demon can come back tomorrow and kill the village in it's sleep, and then the PC's will think twice about just chasing away something that evil agian....
Don't overlook Will-save-boosting things like Iron Will. SR also works sauce against this spell, and outsiders tend to have pretty decent SR. It's also not terribly effective at long range (only things within 40 ft. are affected), meaning an enemy who refuses to close with them (and has ways of ensuring they can't close with him) will be a continual hassle. Also, a good DM strategy in this situation is, as another poster mentioned, to use tiers. Don't let them rest after each battle, assault them with lower-level evil outsiders and gradually ramp up the challenge in the course of the same day, hitting them at the end with something at their EL or slightly above.
Finally, and don't laugh, consider bard levels. Silence, countersong, good Will save....it's the ultimate class for a demon who wants to stay mired on this plane.
PS: remember that they're only gone for a day....if they got to this plane once, they can presumably do it again, and under their own time and their own effort, ensuring that when they come back, they're going to be in a much stronger situation than before. And potentially with safeguards against what sent them away the first time.