I love my medium sized Minnihaha saddlebag in all aspects except it takes away the seatpost as a rigid mounting spot for a taillight. Even though it has a loop for attaching a light, it will be very poorly aimed most of the time (unless the bag is stuffed just perfectly). Even a less directional light than the Hotshot I am using would be poorly aimed.
I have a rear rack (which block seat stay mounting) that has a hardwired light mounted to it so that real estate is spoken for. Would rather have it firmly mounted to the bike than on my helmet.