I have a 5 shaman team set up to do 3 purges/1 windshear/1 frost shock. This removes 6 buffs, snares, and silences, in 1 GCD. I have it set to the control mod of my 5 flameshock macro and use it when EM is down and I plan a LvB nuke on something that can dispell/load up HoTs. Hit 5 FS, Ctrl for snare/silence/buff removal, and then LvB for the kill 90% of the time.
More than 3 purges at a time seems a bit overkill, as the buffs that avoided the first checks for resistance will also avoid the rest. 4-6 buffs being removed should drop anyone to kill range for a 4 or 5 shaman setup. I don't have the purge on a castsequence as it really has no benefit.

One interesting effect of multiple simultaneous purges occurs when going against another boxer. 4 simultaneous purges removes 4 GTs, not just one like 4 or 5 of any other spell cast in a short time frame. So if you go against another boxer in arena purge their GTs then immediately nuke for a huge opening advantage.

Anyway I'm drifting off topic...short form 4 purges at once removes up to 8 dispellable buffs.