1st, it seems Mr. Parlier is trying several new ideas at once. Not only is he trying the biplane rig but he is also using stepped hulls that are very much like seaplane floats. These (if properly designed, which I assume they are) should be very effective in real blowing conditions. In lighter winds, I would expect the steps (which are really shallow wide transomes) to cause an unreasonable amount of drag. I suppose the extra sail area permitted by use of this kind of rig was supposed to make up for that.
2nd, Just because the biplane rig may prove deficient as a RACING rig does not necessarily make it deficient as a CRUISING rig. A cruising boat has different needs than a racer. Ease, convenience, safety, and minimum cost are all primary vertues of a cruiser. Where speed and, to a lesser extent, safety are the primary vertues of a racer. I think it is a great mistake to believe that these two very different types should neccessarily even look like one another let alone have the same kind of rigs.
Think of it this way, you can completely douse half the rig on a biplane rig and still have roughly the same center of sail area in relation to center of latteral area (there by maintaining the same steering ease). This would entail no tying of reefs, no expensive roller furling, and not having to stop the vessel at all. Just a couple of down hauls would do (just remember to douse the windward rig).
This would prove most useful when suddenly caught over powered in an unexpected gust, especially on a multihull.