Ok, this is just my opinion and as far as I know to be true:
1) cropped ears: I think in fighting breeds like dobermans and pit bulls, they cut the ears to decrease injuries to large floppy ears during dog fights when grown (easily torn) and in some hunting breeds for similar purpose. But for a pet or a show dog, that's purely arbitrary and fashion-driven.
I would say much the same about tails. The argument of the modern "hygiene" and "tail injury" doesn't ring absolutely true, due mainly to the fact that many other toy breeds keep their tails as part of their breed standards, and everyone thinks it's "wrong" to dock their tails. So, if chihuahuas keep their tails in the AKC show ring, why on earth must yorkies lose theirs?
2) pain argument: OK, I grant, everyone is able to survive an operation and live a happy life ever after. Docking and ear cropping cause temporary pain, but won't affect quality of life ever afterwards. But it cannot be denied that there is pain: whether a pup yelps or not is not a 100% reliable indicator. It could be that they are too young to give physical reactions.
On the other hand, to subject a pup to uneccessary pain, for fashion, is ethically questionable. If it were a dog for hunting or for working in the field or for dog fighting (which is not so ethical, either), docking and ear cropping might save some very likely future pains and infections. But for a lap dog and a show dog??? Yorkies are no longer hunting rats.
Circumcision in baby boys and cutting off tails and ears is not the same: circumcision is skin deep. A tail is part of the back bone and ears have cartillage (soft bone). It would be more comparable to cutting off fingers, ears, and noses on a human than cutting off foreskin (not that circumcision doesn't hurt...). But of course, even a human baby can survive losing fingers, noses, and ears and live happily into adult hood. Afterall, the Mayas flattened the foreheads of their baby daughters, artificially crossed their eyes (by danging beads in front of their noses for most of babyhood), the Chinese broke and bound the feet of their daughters, and we still pierce the ears of ours... Amputations don't always destroy lives.
I would say: unless fashions would change, we must accept show standards and accept breeder's decisions about cropping and docking: afterall, we can't blame them for wanting to find homes for their puppies. A law in a democratic country must follow public opinion: you can't do it top-down. The populations in the UK and other European countries found it unacceptable to inflict unneccessary pain on their animals. Perhaps they are pioneers, but perhaps they are also only just exceptions. Far more important, though, is quality of life over the entire 14 year span of a yorkshire terrier (or any other pet), although how they are welcomed into the world (with a clip or not) might just be a small indicator. (In Germany, for instance, all animal shelters are no-kill).
But that's a discussion too long for internet forums.
Best wishes,
mbfleur