People wouldn't expect it of me, but I don't think it's cruel, just like cropping a dog's tail (though I think that's tacky).
All 3 of my cats are declawed in the front. No amount or combination of (positive) punishment, alternative scratching surfaces and trimming the claws would save the furniture....and it was real bad. Our couch was destroyed all the way through the stuffing to the frame, while we tried to train it out of them. Also, trimming the claws was a horribly traumatic experience for one of them everytime. She just fuh-reaked out, and it became a dangerous situation for everyone. Ever had a septic hand because of a cat bite? I have.
So we had it done after much delibaration, and honestly, they're no worse off for it. Obviously they're inside only, but if they escape, they still possess their back claws for traction, defense and climbing. They play, climb, run and love on us as much as they ever did....they just don't destroy our furniture now....but they try..ooh boy do they try.