a cross bike is meant for speed on dirt tracks. your not usually racing with stuff packed on a bike. however, it seems that many cross bikes do have mounts to put a rack on. Speed of the bike is determined by weight of the wheels and how aero you can get on the bike. weight of the frame isn't a big deal.

honestly, just stick some knobby 32 or 35 tires on instead of slick 32 or 35's if you want to do the occassional dirt path ride on your commuter. Thats what I just did, it works great.