I'm not a big fan of the look of double top tube frames either, but it's purpose is definitely to reduce frame flex, while maintaing the same tube diameter. This issue can be addressed other ways via larger diameter tubes or sloping top tubes. My guess is that some manufacturers utilize this method as a point of style, giving a nod to "old school" technique. The Pashley Guv'nor uses the same technique. For me, I'd try to make the smaller frame work and address the sizing issue with a longer seatpost (with setback if necessary) and a longer stem.