I did notice that in Mordus Cave, some kills did not warrant XP. It was limited.
Yeah there were a bunch of enemies who were killable in one-shot. And they gave 100 XP. At level 10, it takes about 135,000 XP to reach level 11. So those ones were negligible, only serving to waste your AP.
I am long past that part, so I don't think people are talking about the 100 XP enemies not giving XP. I think they're talking about tougher enemies who should be giving some.
In any case if XP in Reaper's cost needs nerfing, it should probably be trimmed off of the level 15 enemies - not all the way down, but a little from each of them.