When we first acquired our Southdowns our (commercial) neighbour looked over the gate and said, "There maybe a reason why they're a rare breed." Nine years on I told him that we charged the society auction upset price for our breeding stock and he said, "Oooh." I agree it can be helpful to breed something that'll be recognised at the local market. Around here a Jacob crossed with a Down breed is known as a "farmer's cross," because the lamb is so sweet the farmer keeps them to eat for himself!
Lleyns were a failure for us. Ewes very nervous and defensive and sisappeared to the end of the field every time they spotted a human - much worse when they had lambs - and although they scanned with triplets and quads none ever managed to raise more than twins.