Wait till you see a squirrel panic in your living room. You will quickly loose interest in them as soft, cuddly creatures. Raccoons will have their young in the safe and warm confines of your smoke chamber. The droppings from raccoons carry the roundworm parasite. This can be fatal in humans.
Raccoons also carry rabies.
Once again, these creatures that are so fun to see, are best left in the wild, not in our chimneys and fireplaces. A cap inhibits downdrafting. Those nasty little puffs of smoke that all fireplaces belch into the room from time to time, are caused by air coming down the flue and pushing the smoke out. Errant down drafts from windy conditions or the slope of your roof during specific wind direction will be inhibited from entering the flue and smoking into the house. A cap prevents roof fires.