First of all, there are three kinds of defense.
Passive defense:
Relies on so much armor/TaN/Iro/Bad that you can just walk into enemies fire.
Pros: You don't have to play with as much skill, since you can take so much punishment.
Cons: You usually don't have much offense to speak of, relies a lot on armors.
Works well with: Vampyre, Army of the Dead, Survivalist, Entrenchment, Malicious Blades
Active defense
Relies on high movespeed and dodges
Pros: Doesn't rely as much on armor, extremely good vs bosses
Cons: Autohit enemies are your worst nightmare. You don't have the durability to take their hits, and you can't dodge revs and viles.
Works well with: Blademaster, Fireangel, Shottyhead, Gun Kata, Running Man
Tactical Defense
Relies on player knowledge, listening for enemies, and cornershooting/radarshooting
Pros: Extremely low trait cost, you can do this without any traits. Can be used to augment another defensive mode.
Cons: Requires an extremely experienced player to do properly, one screw-up can doom you.
Works well with: Ammochain, Sharpshooter, Bullet Dance, Cateye, Scavenger