I have avoided revealing this, as it is such a heated topic, with good reason. However, both Boo and Drizzle are de-clawed (and spayed and neutered).
Now, as to Why:
First, they are both indoor cats, so they don't need the defense portion. But that wasn't a reason for doing it. When Boo was de-clawed, it was before I was really a cat parent myself. I'd only just had Drizzle a few days. I knew nothing about cats. My sister chose to have Boo de-clawed. I don't know the reasons. However, she too was a new cat parent at the time. We've never had pets as kids because my Mom was "Alergic" :roll:
I don't recall what reasons made my Sis have Boo de-clawed. She'll have to post them. I do believe it may have been because Boo's claws where, literally, RAZOR sharp. She was (and is) a very skittish little girl because my sisters Ex was too rough with her. He had no competence on how to handle an animal. He is an idiot. Right PrettyKitty?

Anyway, I believe Boo's razor-sharpness and skittish-ness where perhaps the focal reasons for the de-clawing. That is what I surmise. It may have been that at the time it was thought to be the standard normal thing to do, common practice. I really don't know.
I do know that Boo has become less skittish since. Also since the Ex became and Ex and not a "current".
I can speak for why Drizzle was de-clawed...
With Boo de-clawed, Drizzle was injuring her during play. It was too harmful and dangerous for Drizzle to remain clawed while Boo was not.
I can't say I disagree with either choice. In my experience it has had no ill effects on either cat in the long run. In the immiediate, did it have some ill effects? Yes. But only in that they had to adapt some things. All around it was the best choice for these two cats, thier 'parents', and this situation.
Now that it's been over and done with for 2 or so months (for Drizzle, many months for Boo) it is fine and dandy.
They where un-able to use the floor-to-cieling cat tower for a while. They where used to digging claws into climb. Now? They've adapted and climb without claws. They still scratch on furniture, lol, but not because of claws/lack of claws, but for routine, excersize, and habbit. Both cats are happier, believe it or not, because when they play they aren't injuring each other. Likewise, it's nicer to not be injured when holding them.
For our cats, and us, it was the best solution and has had no long-term ill effects. It has had benefits, to be honest.
Does that mean it is best for all cats/other cats, or other people? No. Each cat and each person is individual and each choice must be reached as such.
IF YOU DE-CLAW:
FIND AND USE A LASER
When Boo was de-clawed, it was done with normal, standard, BLADES. In hind-sight this was a bad thing. However, at the time, the use of a laser was not known to us except through my sister's Ex, who told us it was a multiple-hundreds-of-dollars procedure, and even then we didn't know of any vets who did it, or any people who had had it done to thier pets. Boo, afterword - being de-clawed and spayed - was miserable for a week. It was very tough on my Sister. At the time I'd only had Drizzle a week or so and wasn't much of a "cat parent" yet lol. It was terrible on her though, and of course, worse on Boo, but it was thought to have been a "nessascary evil"
When it came time for Drizzle... I agreed, because he was hurting Boo. when they would play, his claws would cut her, and it was too risky and harmful to leave his claws in. So, I, now a cat parent, agree for Drizzle to be de-clawed. Thankfully, a Doctor-friend informed me her cats where done with lasers, and that they had suffered nill. They'd been playing and back to thier old selfs within the time it took them to wake up from the drugs. So I insisted we investigate this, hoping we could do Drizzle this way. Sure enough we found a vet that did it for $119 - when normally it costs about 30-40 with blades. However, if it had been available for Boo and we'd known of it, we surely would have done so for her too.
Point is: If you do it, FIND and PAY for a LASER.
No Pain. No recovery. To ill effects. Drizzle was walking that afternoon. He was kneading me on the car ride home. He was fine with it. He was never in pain. I never saw a drop of blood. He was fine. Sleepy, but fine.
Will he "cry" when you leave him/her at the vet? Yes. I cried too. So will you. So do most cat parents. It's traumatic to leave them at the vet, especially over-night. Thankfully, it seems MORE SO for you than them. To them, they forget about it as soon as you pick them up and hold them. They're like "I couldn't find you! Finally! Yay! HUG!" and happy. Will they put up more of a fuss to go back in the carrier and to the vet again in the future? LOL. Yeah. But, point is: it's really not that trying on them. They're more curious at everything new, and a lil confused. As soon as you hold them though, they are over it. Happy. At least, that was my experience.