Bathroom Fan Venting Into the Attic?

I purchased a new exhaust fan for a bathroom that's never had one before. The guy I contacted to install it suggested ventilating into the attic, rather than externally, since the attic itself is already ventilated. Anyone have experience with this? He said it's a very common move, but I want to make sure it's not a shortcut that could lead to expensive problems (mold, etc.) down the road.