Here are a few things that make it better than the bikini frame:
1) with heavy load hauling you will not have pinpoint pressure in the lumbar region.
2) there will be no chance of something poking you in the back.
3) you get the comfort with lighter loads and also heavy loads without sacrificing anything after reaching 80-100+ lbs (if the arrows work for you).
4) the shoulder straps are thicker and more adjustable for taller people on the UL duplex, when in comparison taller guys would sometimes have issues with the bikini. This is the same for shorter and average height people as well (meaning the shoulder straps are more comfortable ).
5) you can swap stays on the UL duplex, whereas with the bikini you are stuck with whatever stay you ordered. This isn't a big deal for most but if you are wanting to run a UL system for backpacking and scouting and then go to a heavy load hauling system later all you will need to do is swap out stays. To do this with the system we have now you would need two frames, a bikini and duplex.
This is not saying that the bikini is a bad option, but keep in mind how many phone calls we field each year about our frames and suspension systems. ...