I use both my commuter bike and my road bike. During the winter, it's solely my commuter has it has fenders and lights. During DST I take my road bike about half the time so I have the option of doing a longer ride after work or going for a quick spin at lunch. I use a backpack when I take the road bike with no problems, though when I get it together enough, I prefer to bring a couple days worth of clothing and lunches in at a time so I can ride in without a pack sometimes.
Good luck!
"Bicycling is a big part of the future. It has to be. There's something wrong with a society that drives a car to workout in a gym." -- Bill Nye