I just happen to fall definitively on the other side of the fence on this one. I tend to already have more cool character concepts than I can shake a stick at. I buy books because I want a specific slew of options, and am very unhappy with that book providing only a small fraction of content that's actually usable.
...
If I'm buying a book called "The Complete Elf" then I know up-front that if I want the bulk of it to be useful to me, I'd better be playing an elf. If I pick up "Martial Power" then I expect the bulk of its content to be applicable to any martial character. Not martial half-elf genasi born in July.