IMO the benefit of box rails is that they break the water contact with the board, hence less water/board friction/drag. in the ocean this works well as the board slices through all the chop allowing the full benefit of the runner beneath. this is why the naish boards work so well. unfortunately the problem i find with these boards is that the rails tend to catch and if your not focused 100% of the time you will be thrown off. fanatic boards seem to have a good balance that works well (rounded front boxy rear) as they are quite stable, just as fast and catch less. personally i prefer and ride DC, they have rounded rails throughout, and for me are the most stable and fastest boards to use. i find the rounded rail is the easiest to use/most stable and surfs really well.
hope this helps