WHAT IS IT WITH all that luggage marching down fashion runways? Last season Miuccia Prada loaded down her waiflike models with rolling suitcases. And at the recent menswear shows in Milan, Valentino outfitted a jet-set couple with their own airport trolley, right, complete with a signature red set (clearly he doesn't intend for this pair to stop at baggage claim). "Luggage is the obvious next step after handbags," says Robert Burke, former fashion director of Bergdorf Goodman. "It makes a lot of money for the designers, and it can stay on the floor for a year, whereas a men's shirt only lasts a few months." It also provides a chance for men to join the accessories fun that women have been having for years without risking their masculinity. Apparently, the key is for them to look as if they're traveling. Function over fashion after all.