I use a lot of BM for winging Spey flies, it's not as "durable" in terms of looking the same as it did day one - but it still catches fish just fine even after the previous fish parted the barbs a little. The general impression of a tented wing in the distinct color and barred design still remains even after several steelhead, if one should be so lucky.
For me a Spey fly that has greaselined one good steelhead is as durable as a fly needs to be. Now if the angler is dredging the fly, that's another story. These flies are meant to be fished in the classic manner.