I think people likes to use LB at the end of the battles just because it is when It's starting to get difficult... When the bosses reach a certain HP they start changing phases, dealing more damage and such things, in the latest phase it begins to become too much dangerous to keep it like that, so.... It is better to finish it fast with a LB...

While, initiating a fight, there's rarely a problem with holding the battle like that... Let's say, normally it's easier to reduce one boss's VIT bar to half, than from half to zero...Yes, maybe the same vit, But it's not the same try to reduce, for example, the first 30k HP of 60k HP than the latest 30k HP....

That's why most people use LB at the end of the battles, because at start has no much use at all, only accelerates the boss's fury.

(Of course, I'm a noob, and I'm just giving my opinion of what I have seen until now.. MAYBE in end-game there are bosses that are harder at the start of the battle instead of the end... Even if that would make no much sense for a MMORPG.)