I have a different point of view. For a 5.5 mile commute on paved roads, pretty much anything will work for commuting whether that constitutes 95% of your annual mileage or 5%.

If for example you find yourself hankering to do the occasional alpine run, then you might want an MTB with a set of slicks for your commute and a set of knobbies for your hobbies. Likewise if you're thinking you'd like to give a triathlon a shot, or a long distance tour, you might want to lean towards a drop-bar road bike. Of course, you could own two or more bikes, and many people here do, but it's not really necessary.

The standard recommendation for a commuter bike is something that will take fenders, a rack, and lights. That doesn't eliminate a whole lot and truth be told, many people successfully commute without fenders, racks, or lights. There's a woman in my office who probably spent a couple of grand on a bike + accessories for commuting. When fall rolled around she dropped another $200 on a lighting system so she could ride in the winter. Well, I don't think she's been on her bike since November so the money spent on the lighting system was pretty much wasted.

I know that a shiny new bike might help get you motivated to commute, but if you don't need that motivation then maybe the best thing you could do is dust off your mountain bike, throw some fenders and slicks on it and commute for awhile on that. Keep note of what would work better. If those hills are killing you, then a lighter bike might be in order. If your hands, wrists, neck, and/or back keep getting sore then may you want a different riding position, or to be able to alter your riding position. Is it windy on your commute? A lower riding position may help. Is ice and snow an issue? Then you'll want something that can take studded tires.