Forgetting the first argument and whatever else has been said in the meantime, I'd just like to reiterate these points.
The main reason that tails are docked from lambs is to stop fly strike.
I think most of you have have missed the most valid reason to shear sheep and its for there welfare.
1. The most important reason is to prevent fly strike and if any of you have seen the results of fly strike im sure you will agree that shearing them is much kinder.
2. Prevention of casting when fat sheep particulary crossbreds have a full fleece they tend to lie down on there sides when they roll they basically create a wedge of wool on both edges preventing them from getting back on there feet. If this happens they are susceptible to crows pecking there eyes out or foxes eating there backsides.
While I don't agree with many farming practices, shearing and docking are two things I most definitely agree with. Having lived in a farming community and having spent a lot of time on farms, I have seen the results of fly strike, and they aren't pretty.
Think of it this way, would you prefer the animal underwent a few moments of pain or an extended period of suffering? I know there are differences between the practices but, think of it like desexing your dog or cat. They're going to be in some pain, but in the long run, it's for their benefit.
That is all.