For me it is all about where im raiding. Certain fights, different specs perform better. With that said, i respec pretty much before every raid. BM does the highest damage in fights where i can just stand and shoot the boss, MM does better on fights like Vashj, and Alar, or where anywhere u can reliably use multishot, and SV is good at pretty much everything, which is why i consider it my main spec. Over all though for ME, the difference in dps isnt that much, 1200-1400 bm, 1100-1200 MM and SV. This is the reason i switch specs, because raiding as a hunter is pretty boring, so to keep myself interested i change things up a bit.