Declawing weakens their muscles
Cats dig their claws into things to stretch and strengthen their muscles. Without them, they're no longer able to work the tissue properly, causing it to degenerate over time. This leads to chronic issues like arthritis as they get older.
It leaves them defenseless
Cats use their claws to defend themselves against predators. Taking away their claws leaves them without any method to protect themselves. Imagine if your kitten got outdoors and couldn't defend herself! It can also lead to bullying from other household pets who are bigger than them.
Cats who are declawed have also been known to use biting as a way to ward off threats which can lead to both strangers, owners and the cats getting injured. The back claws, if they remain, are an ineffective form of defense because they require the cats to lie on their back, leaving themselves vulnerable.