Yes, it was possible to clear content before Shirk.
However, if you take a look at all 3 tanks' aggro combos, you'll see that they're all a DPS loss (Butcher's Block is a potency gain vs SP, but you lose 10 gauge which makes it a loss). So how do we avoid doing those aggro combos as much as possible? By getting as much aggro as we can from elsewhere. In 3.X (and 4.X) this was from NIN and allowed for the most efficient speedkills. Now we have Shirk, which with a proper swap will gain you a ton of free aggro.
Basically, taking Shirk and using it properly is a pretty big DPS gain and when the goal is to kill the boss that tends to be a pretty important thing. Plus there really isn't anything to take in place of it (Rampart/Provoke/Reprisal/Conva/Shirk should be your go-to for 8man stuff, swapping Shirk for Awareness in dungeons).
OT vokes -> MT Shirks OT -> MT vokes -> OT shirks MT. That's for the Shirklejerk, but that's how it works anyway (OT vokes -> MT shirks).
http://forum.square-enix.com/ffxiv/t...=1#post4282644 <- Shirk macro