1) Causing a fire should they reach the electrical wires. Primarily known for their big teeth, squirrels are big chewers and they don’t discriminate what is chewed.
2) Although they invade your attic, it does not necessarily mean they will stay in the attic. Therefore they could end up in your living room and attack if they feel threatened.
Ultimately, the best way to eradicate squirrels from your home is to trap them. However, trapping should be conducted by professional animal removers who can handle it efficiently and safely.
Dinger, Bruce. “How can cute squirrels be so dangerous?”
Accessed 17 October 2010