If a declawed cat got outside, and another cat wanted to fight, he would have no claws to protect himself. They use claws to capture small insects, etc. also. I do think cats sense if another cat is "different"....dunno if they play upon that.
The issue of "debarking" dogs comes to mind also. Some people find it cruel, but in the most extreme cases (such as a dog that will bark at anything that moves), it's the only recourse if they want to keep the dog as a pet. It happened to me, and my Vet was extremly reluctant to debark my dog. So the Vet only "partially" debarked my dog....her bark was very low sounding and not normal.....
IMO, cat and dog owners, if they are truly responsible, and chose to declaw their cat, or debark their dog, if the reasons are valid, and they plan to keep the pet a lifetime, then it's up to them......But things do happen, and they have to part with the pet due to unforeseen circumstances. And if they cannot find them a home, some of the animals are just "put outside" to fend for themselves.