There should probably be a cooldown on changing subjobs in the field. If not, you could end up with people (especially mages, because they don't need to engage the mob) changing subjobs several times during the same NM fight. RDM with elemental seal, divine seal, accession, light/dark arts, stun, who knows what else?

Idk though, sounds too exploitable to be implemented. It doesn't take that much time to go back home to change your sub anyway.

As for changing to /blm for warp. What seems like a better idea for that would be to just make warp scrolls a buy-once item with enchantment-warp and a 15-30 minute cooldown on. They are "free" already, only reason people don't always have one is because they forget to pick one up.