My Cu Chu, Sieg, and Vamp all need the xp and my Valk is brand new today so it's not being wasted at least. But yeah, I went 2 for 3 on Cu Chu and then something like 0 for 28 on the rest (including the 5 Drops I just fed to the Valk I got a few minutes ago).
Btw, PaD forums have collected a ton of data on skill ups and it's nowhere near 1:5 on a 2x skill up event. I can't find the post again, but they said it's more like 1:10. Personally I think different monsters have different skill up %'s. Some, like my harpies that were trivial to get 15 skill-ups for each, are really easy to do while others are fucking retarded, like my vampire that never gains a goddamn point no matter how many big baddies I feed him.