Sloping top tubes also called compact frames design actually increases the stiffness on the bike frame by making a smaller frame. However the seat tube needs to be longer and stronger to compensate. The sloping top tube give greater standover height. Many people find the extra space comforting even if it is not needed. As noted you can make a smaller selection of sizes work for more people if standover is not a problem. Frames are also slightly lighter due to the smaller size. Fit of a sloping top tube CAN match a straight top tube frame so fit is not necessarily a problem. However I think many cyclists find a level top tube to look better. In addition for smaller frame sizes fitting a water bottle can be difficult with a sloping top tube but no problem with a traditional frame.
I think its great that bike manufactures are making comuting/utility bikes but I would like to see the hub geared versions available in the US. I think it is a matter of the perception of the market. Most US customers will not pay more for a bike with less gears so the derailer versions greatly outsell the hub gear versions. Which makes it uneconomical to distribute the hub geared version to the states. I guess the market is different in Europe.

Craig