The bind penalties are there for a reason.
A) Swapping gear would be retardded. You can't go 100% DMG for a nuke but wear as much dmg-% gear as possible. Picking gear a balance between offense and defense is part of the strategy. Otherwise, everyone would be doing the exact same thing.
BLM swaps to DMG gear for Freeze II on a slept target
Slept target pulls some 100+ ice resist build out of his ***
Spell gets off, doesn't kill DD
DD runs for mage to ws
Mage tosses on every PDT gear in the book
Ect, everyone would just be doing the exact same thing, no style to it at all
B) The real reason. Targetting. You can't target someone who is doing nothing but swapping gear. You could alternate two pieces of gear in a macro and just spam it the entire match