An inch over the full is about .75 to 1 qt more than needed and yes its not good to have that much extra. Like ben touched on and also it can cause the oil to foam a lot more because now you have the crankshaft sloshing around through the oil. Like he said it can cause blow-by and so on and puts a lot of air bubbles into the oil and air bubbles in oil is not a good thing for lubrication.