People doing shrugs in the rack pisses me off too. The main reason for that is because the only people I ever see doing shrugs are small as hell and they only do them so they can load a shitload of plates on there and think they look cool. Then when they actually do a set, it looks like they are humping the bar and having a seizure at the same time as they jerk it up about 1" using every muscle in their body except their traps.
If you can't deadlift the weight from the floor, you have no business trying to shrug it. If you can shrug more than you deadlift, you are either using shit form or need some serious help cause that's just sad.
Now if the gym is empty and no one is near the squat rack then do whatever the hell you want in there. But if there is even the slightest, most remote chance that someone might want to squat, get the hell out of there. Not everyone is going to come up to you and say, "hey I want to squat". Some people will wait quitely for the 30 min while you do your 20 sets of bar humps.