Just read this whole thread, thought I would throw in my two cents.
I think that spaying, neutering, etc are in a different category than docking, cropping, etc. You fix your animals because you don't want even MORE accidental babies running around that won't find homes. Sure you can keep an eye on them, but whose to say your lady cat in heat (GOD they are irritating) won't run away to the neighbor's? Or the stallion down the road won't go crazy about your mare in heat, break the fence and come on over? Animals in heat, as well as the studs, can and will find ways to breed. Unless you're 100% prepared for babies at all times, fixing should be done to prevent it.
Cropping ears and docking tails I think is very unnecessary. The ONLY time I can see it being okay is for health reasons - I.E. A long eared dog getting multiple ear infections. There is a reason they look that way, and no reason to change it. Same goes for dewclaws - unless you have a hunting dog, and they are prone to catching and ripping off, why take them off? There is no reason to.
Declawing a cat, I also think, is unnecessary. Amputating the last joint on their paws is a cruel procedure that they shouldn't have to go through. Unless there is a health reason, all of my cats will keep their claws.
And the last one.. Debarking a dog. I think debarking a dog is THE most abhorrent thing you could do to a dog just because you don't like that particular aspect. At least with their ears cropped, they can still hear. Tails docked, they can still wag. But if you take away the voice of the dog, that's wrong. Taking out his means of communications because you don't like it is outrageous and I am 200 percent against it.
There are times - certain medical issues - that can validate these particular things. But if you like barking, DON'T get a dog. If you don't like floppy ears, get a pointy eared dog. If you don't want an animal scratching up your curtains, don't get a cat - that, or teach it not to. I've done it a bunch of times. The key here is to get an animal that fits your lifestyle, NOT to get an animal and then fix it to match your lifestyle. You don't all have to agree with me - Everyone has their own opinions. That just happens to be mine.
AS for the original post - I can see why it used to be done -to not tangle in with the driving lines, but I personally would never dock my draftie's tail. If necessary I would braid it, or if REALLY necessary I would just cut the hair shorter. But why dock? That would mean the tail could NEVER grow back (Right...?) So, when the horse retires out to the pasture, they have no fly protection. Not to mention a nice, long tail looks beautiful!!