You're right about they can't stop class stacking, but they can discourage it. Something they haven't done yet.
One of the biggest advantages ARC has in the moogle fight is their combos are a joke to pull off. Unless the mob is aggroing you as an ARC, you don't have much of an excuse not to be 8+ yalms away from the target. However, the same can't be said about melee. Comboing the moogles as a melee is somewhat annoying due to their size, them running away and calling for help after they take so much damage, and possibly latency registering the hit in an unintended direction. I've noticed if I run through a mob and use Impulse dive without pausing for a moment, the client registers it as a frontal attack even though I was behind the mob on my screen.
One of the best remedies for this would be removing the crap requirements for combos and having them only fail if you miss or get resisted. Even then I fear people will only use archers, claiming MRD/BLM AoEs do too much damage or some bullsh*t excuse like that.
Stupid excuse? Very. But...
