actually have a slightly sloping top tube.  My 58cm 'Dale has a 56cm c-c seat tube and it does slope slightly.  In the smaller sizes the slope is much more apparent.  I would characterize bikes into 3 groups:

1.  traditional (level top tube)
   
2. Sloping.  Standard sizing but with a sloping top tube (Cannondale, LeMond, Felt to name a few).
   
3.  Compact- Generally sized in XS-S-M-L-XL with "virtual dimensions". (Specialized, Giant)

the Cannondale sloping design works well for me since I get a nice roomy 57.5cm top tube for my longish torso and a friendly standover height (56cm seat tube) for my shortish legs.  
Compacts allow the builders to limit choices since they take standover height out of the equation so they only need to focus on the virtual top tube length which can be tweaked with stem length, setback of the saddle etc.  
For comuter bkes I would look at sloping or compact since it's nice to have extra standover height for commuting since it involves more forced stops than the wekend club rides.  

When you make the top tube slope, both the top tube and the seat tube are shorter, which makes the frame weigh less. I think that is the main reason.

If possible look around for used/older bikes for a commuter (and something that will take fenders and a rack).

Older mtbs (hard tails and rigid forks) make pretty good commuter bikes unless speed is an absolute necessity.

sorta tackles this subject in his latest tech column at Velonews.com.  In response to a reader question about the proliferation of S-M-L sized frames, Zinn points out that sloping top tubes have eliminated one of the factors that caused people to think/realize that a frame is mis-sized for them, thus letting manufacturers get the economic benefit of fewer sizes without annoying consumers.  More info there, if you care to check it out.

 

Thanks for the responses.  Do many commuters or other bike riders own more than one bike?