I've been reading up on sheep breeding - not because I'm planning on becoming a shepherd, but because I'm interested in how commercial sheep herds are run. And I've learned something interesting about the Bluefaced Leicester breed as a result!
Turns out that many commercial herds - and I'm talking about the meat sheep herds here - are not purebred. There is a very successful breeding program, developed in the UK, called a "stratified three-tier system", which produces very good market lamb. About 50% of the commercial ewes are part of this system in the UK, so I imagine that quite a bit of fiber comes from this system as well. In north america this system is starting to catch on as well.
The breeding program works like this:
Start with a purebred flock of ewes that are suited to your landscape - in the UK, these sheep are typically the hill or horned breeds like Swaledale, Scottish Blackface, Cheviot, Welsh Mountain. Get a Bluefaced Leicester ram and breed him to your flock. The female offspring are called "mules" and are very productive - they have improved fleece, better meat, and higher lambing rates than their purebred mothers (the male offspring are not mentioned so I'm assuming they go to slaughter, although they won't get prime prices). These mules are a product in their own right, and you can sell them to other farmers. You can also sell your fleece from the purebred mothers. The BFL ram is key to this operation - the whole breed was apparently developed for the express purpose of this type of cross-breeding.
The mules form the base of a meat-sheep operation. They are further cross-bred with a so-called "terminal sire" - a ram with high meat-bearing characteristics like a Texel or Suffolk, and the resultant lambs are then sold as prime meat. The "terminal sire" is chosen specifically to obtain particular meat characteristics: muscle mass, optimum size, etc. Some breeds are suited to the "hot house" lamb market (35-50lbs), some to the "ethnic" market (50-100 lbs) and some to the "mainstream" lamb market (100-150lbs). Parasite resistance in the terminal sire's breed is also important if the lambs are going to be grass-fed. The fleece from the mules can be sold as well, but it's not typically high-value and it goes mostly to the "wool pool".
So the upshot is that there are some shepherds around who concentrate on purebred flocks for producing replacement mules and purebred breeding rams, and then probably more who have cross-bred "mule" flocks. The former would be the source of top-quality purebred fleece, and the latter, of prime lamb meat.
What's interesting is that I've started to see fleece from mule flocks being marketed, as well as lamb from "heritage" purebred sheep (also here). Shepherds need to get as much profit as they can from their flocks! It is hard work, I'm sure.