Zergs started making their way in to exp in late '05, after the fall of Rng. Everyone who jumped off the rng bandwagon suddenly had to go back to the non-rng style of WS WS SC MB and found it to be incredibly slow. I remember the first time I had a non-SC party, around 65 in bibiki bay. I had just started doing Dragon Kick to open for light in my previous party, so I was expecting as much this time around, but instead I was met with "Nah, just go balls to the wall and ws when you get TP". Totally blew my mind, but it was a lot less stressful. By the time I hit 75 it had caught on as the more popular method for exp, though SCs were still fairly common (this was just after ToAU came out).

Zergs in End Game are totally different though, and are something that requires a lot more work and preparation than you guys give them credit. No way in fuck could Joe-shmoe AH Melee walk in to a zerg and be of ANY benefit. The reason they hadn't been used before they were was because they weren't even POSSIBLE before. Hitting the haste cap was a pipe-dream for some jobs, and utterly impossible for the rest, due to the relatively limited gear selections at the time. After ToAU came out and made haste gear somewhat more readily available to the stronger zerg jobs, and we got Cat 2 merits, and such, mages started being more well informed about their abilities, people started getting bored of the old slow fights and decided to experiment, and found something that worked.

Don't you remember how unbelievable it was the first time you heard about someone zerging kirin in under 2 minutes? I thought it was a hoax. Then I saw the video and shat myself, lol. Then other groups tried to copy this strategy, and failed miserably. The vast majority couldn't do it, because they didn't have the gear and experience to pull it off.

Fast forward to today and zerg strats are fairly common and easy to pull off due to the higher levels and better availability of gear.

Personally, I enjoy both styles; long and drawn out fights where you slowly whittle down the NM while trying not to be annihilated, and also NMs where everyone just fucking goes apeshit after getting every maxed out buff possible and trying to kill the fucker as fast as humanly possible because any other strategy just doesn't work due to how insanely obnoxious the NMs TP moves are or due to time restraints, examples being Dynamis Lord before the overhaul, and pretty much every boss in Einherjar.