Also good points. What of those who don't feel it IS an exploit, but actually "the way it's intended to work"?
I see it possible (though I haven't experienced it "live") that a sincere player might think it's meant to work that way.
I see it as a minor advantage that the warlock can make use of when circumstances allow for it (by "it" I mean hexing a creature after a long rest, reducing it to zero hp, and then immediately taking a short rest to regain your spell slot). It's by no means something that the warlock should expect to be automatic or always available. I put it up there with getting a critical attack - it's nice when it happens, it's certainly helpful, but not something you can count on getting for every fight.