It prevents the player-run economy from being flooded though; said resource items are in constant demand across the realms, especially in a game where you're limited in what professions you can take (versus FFXIV that allows you to take all).
Of course, it doesn't stop them from having other means of procuring said materials; it's a very, very large world and they don't necessarily have ores/herbs that fill 7-8 different inventory slots per expansion cycle. MoP (and I believe WoD) also have other means of obtaining said resources that don't require gathering.
FFXIV's shards/crystals run in a similar sense where it's a constant demand, but everyone can gather it, there's no limit to how much it can be gathered in one given time, and the market gets flooded by it.
I would not say it's the case, at l;east not the primary one as of now. Even motivation to do A3s has been at an all time low before 3.1 came out. It'sj ust way overtuned to expect a typical static to clear, let alone have the patience to slam their heads in it for the past few months.
This is exactly what they want to avoid though. You saw what happened with Paladin for Savage in general, it's a piss poor choice compared to a DRK who is essentially better in every aspect. Homogenizing the utility of a job isn't a problem, and realistically fights should not be tuned to cater a specific party composition outside of the norm (it should be perfectly capable of being cleared with 2 tanks, 2 healers, 4 dps and not have outrageous outliers for taking one job over another)
Until they make job exclusive acessories (RIP ring armory), this is going to be the case to prevent players from gearing faster than others. And even then, the armory system is a farshot better from other MMOs that require you to create an entirely new character to play a different job. I honestly think it's fine as is; players shouldn't feel the pressuring need from the game to gear up all of their jobs. And even then, that's what the catch up patches are for, for players whom have geared up their mains already through eso and raids.