"The next big benefit was wind range. Initially multi-fins were knocked for getting planing slower than an equivalent single-fin. But then people realised that, because multi-fins were so much looser than single-fins, you could actually ride a bigger board. Not only that, but at the top end of the wind spectrum, multi-fin boards offered more control. They kept the board manoeuvrable underfoot even when the sail was overpowered, and at the same time, more settled in a straight line with their planted ride. Compensating for slower planing by opting for a bigger board and offering more control at the top end ultimately translated to a waveboard with more wind range – and that’s a very big benefit."
Source: Boardseeker Mag, Multifin wave board test.
So quads have made wave boards much better: tighter turning, better upwind, more control, more wind range... and though they may lfeel slower, testers say its not clear that they have a speed disadvantage (against single fins).
So what are you board designers waiting for? Or may be I'm unveiling some secret race/freerace multifin design?