I have been playing with a ARCH/DEV blaster, and right now he's level 12. I'll admit that I've been playing almost completely solo at this point, and I play on Heroic. I don't street sweep much, and I usually get all of my levels and experience just from playing missions.
From my own experiences, defiance doesn't play into much, as I try to kill the enemy before he can even hurt me. However, against an orange lieutenant, or a red boss, or a large group of white minions, defiance has on more than one occasion saved my bacon. It has allowed me to finish off a boss or other menancing character before he finishes me off. The damage is very noticable.
Defiance isn't part of my strategy per say, but I would hardly call it useless.
