Nah you only skip P3, if you coil Karam he is no longer targetable (not sure if he still gets damage from SE) then about 15 seconds later he casts Purgatory for the second time, heals back up to 60%ish and after that he will still chase you with runes and hands, so you still have to do the hardest phase. So in the end it's actually pretty pointless to interrupt Purgatory if you're not going to zerg Raest.
Anyway, got it down last night on my second try, finally have all 3 done
Edit: I get your point on the exploit, I'll see if Blizzard feels like doing anything about it, personally I don't think it's fair that Cripple only works for the PvP duration while Frost Mages can slow him for days.