Except weapon, all the gear are shared between Blm and Smn. why not people just play both class and use Smn for single, Blm for AoE. Then you would end up being good at both.

I don't see why Blm people need to be only Blm. I play Pld mostly, but when party ask me to go War, I can easily change it. If you have enough time to gear up End-game dungeon like coil then you should easily have enough time to level 2nd class.