Another issue with just buffing the casters is that MCH and BRD are already far behind the casters in DPS. People usually try justify this was 'But they have support skills!' - however, MCH is the lowest DPS of all the DPS classes, a full 20% behind SAM, and even behind BRD and MCH doesn't provide nearly as much support as BRD. We're already in a situation where nobody wants MCH in bleeding edge raids. If you push the DPS casters even further ahead of the martial ranged classes, you're going to be in a situation where BRD is only invited when people REALLY need the buffs and MCH gets /votekicked just for daring show up.

But if you buff the ranged classes AND the caster classes, you've reworked most the DPS classes in the game to get to that point, which bring us full circle - That RDM and SAM are simply punching above their weight and need to be brought in line.