Tony. One other advantage, they are faster to tack so encourage tactical racing. And yet another one. They have more variety of sailing modes. But most do feel pretty sluggish sub planning. In France there has been more racing and there the Pacer and , especially, the Bic 310 have proven top dogs. The Pacer is available in a lightweight construction too, which would be interesting to try. The Bic isn't although most seem to rate the shape most highly. Apparently a guy in France has had a custom copy built at 12kgs, might really show the concepts potential. The Prodigy is in a slightly different class, but the rest have disappeared. As would the RSX if it lost the O status.
Also in response to "the assymetrical spinnaker is something a board does not (yet..any ideas??) have." I've thought about this and came up with the idea of uisng leading edge slots http://en.wikipedia.org/wiki/Leading_edge_slot
These allow a greater angle of attack, which should mean going deeper. Of course the sail would have to have structural integrity with some 'cut out' near the leading edge - not easy. Another problem is a slight loss of windward point due to lesser performance at small angles of attack. But theory and practice suggest the gain could be significantly larger than the loss (not that I'm an expert in the ield - just interested). However in course racing yourer bascially lost if near the back at the windward mark, so proabably wouldn't be worth the effort. Would be great to see it tried out for 'downwind dash' racing though...
Actually the longboards are pretty fast compared to most boats in many situations- the IMCO was rated the third fastest class behind the Tornado and 49'er. Never had the chance to go on a Melges but I know they are a pretty full on race kit too.