It's true sponsors probably prefer their biggest stars to ride the Tour but like they couldn't tell Contador to skip the Giro (even though it was obvious it would mess with his Tour) or they can't tell Rodriguez to not go for Giro/Vuelta (alright, now Menchov is bigger, but it wasn't like this before) or they won't stop Gadret from saying that he likes the Giro much more than the Tour (despite being a Frenchman riding for a French team)- they wouldn't have such power over Andy. Bruyneel has a say in this and he should tell the Schlecks that it's good for them and either they step up in other GTs or they'll have their careers tainted with being eternal bridesmaids (Frank not so much because I'm not sure he has the talent of a GT winner). Unfortunately Bruyneel himself shares the philosophy that nothing but the Tour matters and I'm not sure if he's able to see the bigger picture (in that particular case).
And as for Andy, he won't use his power to ride whatever race he wants because possibly if it depended entirely on him, he'd only ride the Tour and Liege-Bastogne-Liege. The rest of the year he's haunted by bad weather conditions in which he can't race, tooth problems, his responsibilities as party boy and such things. He says "Giro will never be the Tour" and I doubt he can bring himself to really fight for the pink because he doesn't want it as much as yellow and he even has some problems fighting for yellow (I mean his lack of attacking). The only way I see someone could convince him to ride other GTs is if he understood that the experience would help him win the Tour one day... or maybe even this wouldn't work.
But who cares. I'll be happy if he never wins another race and in the meantime I'll enjoy others really doing their best. And back to Vuelta, I guess it's somewhat fair for climbers to have a GT designed with them in mind, considering what the Tour will be like but... too many uphill finishes. I can't wait for some of them (especially the new ones) though.