That's the point. All creatures natively immune to mind-affecting (except Vermin and weird things like the boneleaf or the shedu) do not sleep, and hence can't hurt it back. It is a two-trick pony with the two being almost exactly complementary to each other. Honestly, an Ardent with Practiced Manifester or a Swordsage can get up to 4th level powers or maneuvers quite easily and dish out touch-ranged death on these guys with no way for them to strike back. And for the rest, well. Death Urge is a good power, and you have other quite good SLAs, even if you don't want to just put them to sleep. Or, in the case of a mind-affecting immune creature who can sleep, you can still make a great job helping your teammates by dispelling whatever they try to do.
After looking, in all reviewed creatures (not counting the Dream Phane itself), there are only 9 non-vermin creatures who can sleep but are immune to mind-affecting effects. That's really not that much.
I agree with that. "Not killed but stuck on another plane until it recovers" is not much better than "soul-bound so that it can't be resurrected"