Here in Seattle I see all kinds of riders on all kinds of bikes. The building where I work has excellent bike-storage facilities and showers, so people ride everything from cheap beaters to (very) high end racing bikes. Also 'cross bikes are very popular here as commuting bikes. I think this is a personal decision, depending on what you like.
Personally, I like having:
- permanent fenders
- plenty of capability to carry things in a pannier so I am not carrying anything on my back
- a more upright seating position so I'm up and looking at traffic rather than hunched over
- a bike for commuting that I can also use for touring
...leading me to a dedicated commuting/touring bike.
However, I've ridden my "road bike" to work a few times, with my stuff in a messenger bag, and it's great fun. I see why people do it; especially who ride long distances, who use their commute as a training ride on long days, and/or don't have a lot to carry.