so many classes have so many passive buff procs that can be purged it is often hard to get hand of prot off in under 3 globals anything from resto druid heals, to disc priest passive self procs, to destro lock nether protection and backdraft, to even expose weakness on survival hunters
and like I mentioned before, the real kick in the face, is it removes the MS debuff. sure you have 1 shock, but as BM you're scatter less, wyvern-less, silencing shot-less...
you really only have... HoJ if your pally is REALLY close, arcane torrent, if someone is a belf and up close, or 1 shock. MAYBE WHO KNOWS HEX? if you didn't use it earlier to help pressure and get the focus down low enough where a bop was required.
you're being way too "WELL SHAMAN AND HUNTERS HAVE THIS THIS AND THIS TO COUNTER THIS ONE SINGLE ABILITY" without considering the 0493204930 other things that can be going on, and most of the time are going on in arena.
it's not so night and day as you put it.
it takes several purges unless you're lucky to hit that bop, by this point you've exhausted your ONE interrupt, if it wasn't used prior to the BoP keep in mind shocks sometimes cause pallies to BoP... and the MS is gone, target gets healed pretty hard.