Debris are not a problem, a B-Wing can still be dangerous even loaded with stress. It is when you need to play the arc dodge game with someone that the stress kill you.
The problem with Debris for BBBBZ isn't that the Debris is particularly detrimental to the B-Wings (it isn't). The problem is that the Debris won't stop a PWT from shooting. When five ships are effectively blocking, they can often steer a large based ship on to an Asteroid and cost it a valuable round of shooting. Replacing three of those Asteroids with Debris makes this much less likely and improves the chances of the two ship list.