Sounds like you'd end up with a lot of groups/people choosing to exclude various classes once they found a "optimal" way to farm everything in the fastest amount of time. Maybe if there were optional side-bosses that dropped those things, but had more mechanics or were more of a challenge? Mob-wise, I'd think just having them drop the occasional allagan piece, or a wider variety of materials (or bundles of them), would improve the reward for killing them. At least that way there won't be as much "LFG, ___, ___ and ___ need not apply."