The toes are part of cats' basic locomotive anatomy. When the first joint of each toe is amputated, they have to change their walking angles. You know how when you hurt a leg and start to favor it, sometimes you wind up with other pains as a result, such as a sore other leg or back or hips because you're walking differently? Declawed cats have to walk differently all the time, and are more likely to develop arthritis and to do so sooner.
Further, because declawing initially gives cats wounds on each foot, if they step in cat litter before they're fully healed it tends to hurt. That can lead to litterbox problems, which unfortunately can lead to cats being dumped by their owners, or euthanized.
A friend with a couple of non-declawed cats and a dog once took in a declawed cat who needed a home. The dog barked at the newcomer, who took a swipe at the dog's nose -- and you could _see_ the dog realize that whoa, that didn't hurt -- here was a cat it was safe to annoy! It took a while to train the dog to leave that cat alone.
I'd only be hesitant about an outright ban on declawing because some cats routinely scratch themselves until they bleed. If that's allergy-based it can usually be treated by a change in food and/or litter, sometimes with medications, and it's important to talk with a vet and try all of that first because no animal should have to feel that physically itchy all the time. If it's a purely behavioral problem, however, it can be much harder to stop. I heard once of a cat who could only be stopped from that self-harm by declawing, and that the cat actually calmed down significantly after the procedure.