Alliance only need 11 in WSG because 11 is the soft cap against everything but Night Elves. They need 15 in Arena because they might be fighting a Night Elf.
Ah ok, i have 14 in WSG, then in arena i swap a ring out for the Hit rating ring and swap my bow for the hand canon (balences what was lost from ring a bit) for a 15 rating total.
Well if you want the real hit cap you will need 24% hit or in other words 51 hit rating .
But for normal use 11 if your ally and 15 if your horde. However going above this cap is not really a issue and in some cases might be preferred over other stats.