Tanks can't keep aggro.

[FunkyBunch]

Well, to each their own I guess. I view the BLM rotation to be painfully boring with speed runs as well, but I never bothered to level it all the way up either. But fire II, fire II, fire II, fire II, transpose, blizzard, etc wasn't my cup of tea. I'm sure at a higher level, there's a flare someone in there.

That's not how it works... at all. The AoE Rotation is Fire III, Fire II, Fire II, Fire II, Flare, Convert, Swiftcast, Flare, Trasponse. But not much kills faster than that.
Pre 60, the BLM rotation is a little mind-numbing (F3->F1,F1,F1 over and over).
But once you hit 60 it's bloody hard with all the timings and juggling as your two most powerful spells have two buffs you need before they can be cast and they run out at different times and you have exactly enough time to do the whole rotation if you're perfect. (Dodge an extra AoE or be a half-second late on a cast and WIFF there goes 5k damage).
I routinely grab hate from tanks. Not on purpose, but because tanks don't generate enough hate. If I target one mob only, it takes forever to get a group of 5 down. If I do the AoE damage combo, bad tanks will lose hate, and then I'm the tank for two or three of the mobs.
Thankfully, I can burst one of them down, but it's still a DPS loss if the tank loses enmity on one mob.

[Malicewolf]

Snip

As I stated, I did not level BLM all the way up, so I fall in your mind-numbing pre-60 category and I know the rotation gets much more difficult later on. And in this case, I'm purely talkin about the speed run rotation in which I'm looking to burn a large mob down in a dungeon.

And exactly. Bad tanks will lose aggro to a GOOD BLM. But a good tank who knows how to do his role will hold. Because if a bad tank tries to speed run and pulls too big of a mob with a healer good enough to sustain him, he will eventually lose hate as his TP and Mana aren't unlimited. Unless the DPS are also exceptional, he will run out of resources prior to the mob being burned and then it's a sh*tshow. But for clarification, the point I was making was directly in response to bardaboo's post. To say a tank just spams the same thing over and over and over and use that as an excuse for that job being "easy" is silly. It'll get you by for a little bit and that's it, but it won't sustain you through the harder content.

And I really hope your example is an exception ._. Because a tank should be trying to re-establish hate over a mob he loses.