I don't really like riding too upright. I'll pick my cyclocross over my cruiser for everything except liesurely trips to the ice cream shoppe. It's a combination of all the stuff that's been said already, but I find that on my 6-7 mile commute, if I don't move my hands around they get uncomfortable, and if I sit with too much weight on the saddle, my butt gets really uncomfortable. What I really like about drop bars, especially with cross levers, is that it offers multiple postures, giving you a choice of not only how aero you get, but IMO more importantly, how you distribute your weight. I'm not sure I'd feel up to riding 18km on a bar that only allowed me one hand position and one posture.

Then again, this is what I've found works for me, and it may not work for you. The best thing to do would be to borrow some bikes with different postures and take them for really long rides and see what really works best for you.