Honestly, at first I was going to agree that 90% of the time is better then 100% of the time. Because it does add a bit more challenge (and chance) to the game.
That being said, after reading the comments from other folks, I tend to agree that instead of the frequency of drops (90% vs 100%), that maybe
varying the amount of health obtained from the orbs that are dropped might be a better option.
That way you are still rewarded for slaying the monster (you get something all the time, instead of occasionally getting nothing), BUT the reward varies.
Just my 2 cents, either way I don't have a problem with it.