Nobody does this because shanks (the bit below the knee) need long slow braising. If you leave it on and roast it it’s just not that nice. It doesn’t really get eaten either because the thigh is easier to carve which leads to . . .
2. Waste.Carving a leg of lamb once it’s cooked is tricky and you always miss some – not just the shank but all of it. It’s expensive and yummy so don’t waste it. This is like carving a steak – you eat the whole thing.
3. The bones.You can make stock from the bones after you’ve cooked and carved a leg of lamb but you won’t need that stock for a while. If you butterfly it you can use the bone at the same time to make a stock for gravy for the lamb you’re about to eat.
4. Rubs and marinades.You are exposing much more surface area to marinades and rubs instead of just the outside so you get much more flavour into it.
5. Time.It takes hardly any time to cook it – 30 minutes at the most – because it’s pretty much a massive steak at that point. It’ll go on the barbecue too.
6. You can cut a piece off.Roast lamb is lovely but there’s enough meat to leave some spare. Like in this picture, the bit on the bottom right will be in a Massaman Curry soon!