Tails are generally docked where there is risk of flystrike - it's much harder to keep backsides clean, and much harder to see flystrike, when tails are long.
The problem with not castrating is the need to be able to keep tup and ewe lambs seperate - and the tup lambs away from their mums, too - later in the season. Even where you expect to be able to fatten all your lambs before their hormones kick in, there'll always be one or two go lame or sick and end up staying longer than you'd planned. Or, if you plan to sell them as stores before they get old enough to become hormonal, there'll be a disease outbreak and nothing will be able to be moved except direct to slaughter - it happened in 2001 and again in 2007.