One aspect of Ego Whip that people tend to miss is that it is probably the lowest level die or die effect in D&D. Against groups, as has been mentioned, it's not so good, but against an individual, once it is pumped up a bit, even the half damage from a succesful save is enough to drop many characters.
And then there is the daze. Someone misses a save, they not only lose a big chunk of Cha, they lose their next action as well. Without the half damage on a succesful save, it's in the same category as Sleep or Hold Person, so I suppose that would be OK, but with the half damage, it's just too strong.
In most campaigns, lots of opponents are immune to mind-affecting things, so that helps to balance Ego Whip for PCs, but for NPCs Ego Whip is far too good.
Are you using the suggestion in Complete Psionic that NPCs should be treated as though they had only half the PPs?