the guy seems to just be a pretentious roadie
almost no one who seriously commutes does so on 23's (it's not cm, btw) city riding is just too rough for most. if you look at ANY bike that is sold as a "commuter" it will have larger tires on it (compared to a true "road" bike)
if you are commuting in a city, you don't need THAT much control in cornering. you probably aren't going to be flying through turns at 30+ mph, i consider #2 a mute point
the difference between riding a 'cross bike with slicks and a road bike in terms of one being "slower" is SO negligible that any other number of factors will much more greatly affect the outcome of how "fast" you are, i.e. what you ate, how much water you've had, how much is in your bag, how many lights you catch, wet roads, etc etc etc etc
in conclusion, and i believe anyone in the commuting forum would agree, a 'cross bike is FINE if not preferred by many commuters. what bike shop is this anyway?