They are quite destructive. My coworker did hours of research online and found that once they set up a den on one side of your attic then next step is to set up a latrine on the other side. As you can imagine that cant be good for your insulation and drywall. That doesnt even factor in the damage they cause to enter the house, which you cant repair until you have them out or they will just chew through again. Also they drop a stupid number of pups per litter and, IIRC, have 2-3 litters per year. Its quite difficult to uproot them because once they have settled on a "den" they also become very territorial, meaning he himself could have been attacked/bitten just going in his own attic. Granted, he did it the hard way. He didnt want to kill them so he did the cages, buckets of ammonia in the attic, strobe lights, etc... and it took him about 3 months to get it under control. I would have just lobbed a gas capsule up there and picked up the bodies in a few hours. The down side to that method is they will burrow under the insulation to die (hard to find) so now you have decaying squirrel in your attic.