Looney deserves a break. But looney doesn't guard the perimeter. What does looney being tired have to do with literally every perimeter player we have being garbage at defending it?
It's very obvious that Looney is tired and it affects his ability to battle bigger players and his ability to get tough rebounds. If he gets in foul trouble or defenders have to help down on him, it will eventually affect their perimeter defense.
It's often said that defense is about desire. If you're running on fumes by the all star break, you won't have the same desire to play defense. That's what I see anyway.