I would liken this question to:
When is amputation appropriate?
Only when medically needed for the well being of the cat.
Declawing is amputating the end of the digits (fingers) of the cat. As already mentioned here, it can result in continued pain and can affect the cat's gait. It also renders the cat almost defenceless, and disabled in the cat's inability to climb trees and the like.
It is illegal in many countries.
When making a decision about whether a procedure should be done on a pet, one must ask this:
Is this for the pet's benefit or to convenience the owner?
Of course it is not quite so black and white, as desexing pets, can improve pet behaviour, but as a whole it benefits the pets, from unwanted litters and ultimately homeless offspring.