I just read this article in Bicycling. :

'Compact frames, identifiable mainly by a sloping top tube, are said to be lighter and stiffer because of the resulting smaller frame triangle, and more comfortable due to the longer steatpost, which flexes to absorb vibration.

The biggest benefit may be a better fit for people with short legs and long torso's, because compacts have a lower standover while maintaining standard distance from the saddle to the handle bar.

Does it in the end, come down to style? 101bike.com