I remember looking at the Thorn Nomad mk2 and it had the similar low top tube, I thought at first I was looking a bike that was for a much smaller person but then noticed it was for people of my height. I think my concern would be that the seat post would act as a lever on the frame causing a lot more stress then a 'normal' bike, how true this is I don't know?
I have a similar configuration on my Kona Sutra. The Seat tube has a slot cut in it with a round end at the bottom (sorry no picture to hand). The idea is to disperse the stress and avoid the tube splitting. At least I believe this is the case, others may point out I am talking rubbish