Garlaige Citadel is a hellhole. Others have already pointed out the problems with this zone.
So why is it so popular? Probably because everybody thinks that that's where you're "supposed to go" when you hit level 32 or so. Never mind the fact that Vana'diel is a huge world with lots of areas full of mobs of varying difficulty, and that at any given level there are plenty of places to level efficiently. No, you're "supposed to go" to Garlaige, and if you don't, then you're offending the Brady Strategy Guide gods. This is why everybody thinks that they need to go to the Valkurm Dunes at level 10 (a
horrible idea) and why EXP heavens like the Maze of Shakhrami are now virtually empty.
There are entire zones that used to be used by EXP parties that are now basically just nuisance areas that you have to run through on your way to Valkurm or Garlaige or the Crawler's Nest. Anybody who's ever leveled off of Zus in the Buburimu Peninsula knows that there's great EXP to be had there, but very few people are willing to try anymore. Buburimu used to be "the Windy Dunes"; parties would fight the crabs and pugs there just like they would in Valkurm. Not any more, though. They go to Valkurm, because that's where they're "supposed to go."
If you haven't seen this, here's a great MysteryTour link that shows all sorts of zones and mobs that can be used at various levels:
FFXI Leveling Locations If you can put together a party of people willing to try out a new spot, the results will be well worth it. Not only will the EXP be better (better camps and less competition for mobs) but the drops will likely be better as well. My last mid-40s job had a party that I took to the Labyrinth of Onzozo to fight the Tier V gobs near the entrance. All those Absorb scrolls and fire crystals were worth a hell of a lot more than the earth crystals that drop off of Soldier Crawlers in the Nest.
You've seen the light. You know how well "alternative" leveling locations can work out. The hard part is convincing the rest of your party. When my BLM was in the mid-30s I suggested that we go to Behemoth's Dominion and fight Gaylas, and the rest of the party reacted as if I had suggested we dip toddlers in boiling animal fat. Sometimes it's going to be a losing proposition. Other times, you'll get people with enough of a sense of adventure (really what this game is supposed to be about!) to try something new, go someplace they've never seen before, and EXP off of mobs that they've never fought before.
These parties generally yield more EXP and lucrative items, but the big thing for me is that they're usually just more fun and rewarding. That's the payoff. It's why I play this game. It beats spending days on end in the Crawler's Nest. You won't always be able to convince people to do it, but that doesn't mean that you shouldn't at least try.