My vote goes to something with 2 to 3 wheels.
All a commuter means is that it regularly goes from one place to the other.
It is up to the rider how and on what he\she wants to get there.
I have a aluminum fixie and am currently looking for a MTB beater for winter.
I also plan on building up a road frame that will take fenders and a rack. Not necessarily a touring bike but more like a relaxed roadie. Something like a soma smoothie ES.
In summer I commute on my full carbon bike from time to time.