It's not that AF doesn't support decent damage...BLM isn't just about your potential damage, you get resisted ONCE, and it can mess up your whole spell set/Mana pool. It's the resists BLM's are trying to avoid by upgrading their gears.
I think it's honestly a little ridiculous that gear the game colors blue for very high quality gets stomped by common gear with multiple materia. Sure, it does cost a lot to get multiple-melded items, but must blues and greens aren't THAT much better than other items and they can't take materia at all. Of course this is rather off topic. And AF sort o "defines" a job's appearance. When you're wearing the AF, you look like a black mage and people can regonize that by looking at you. Generic gear for non-heavy melees (The generic plate armors match the classes that can use them better than most) doesn't have this effect- you could be anyone, a mage, a crafter, a DoL. The only real exception here are cloaks but there are very few of them in the game right now.

Weaken BLMs Defense, pull hate you die problem solved. SE also needs to implement class/action specific enmity generation to increase the risk of tier 3/high damage spells/skills.
BLMs are already physically weak enough. They can only survive anything when they use sanguine rite. BLM can't set Sentinel.