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The problem is that the valves move towards the piston, and if you had like a shim over bucket setup with the cams acting directly on the valves, you still run into either the cams not running planar to the crank (and the lobes would have to rotate independently of each other), or you'd still have to have rocker arms that pivot in plane with the valve travel.
They probably looked at a bunch of ways of doing things before they settled on this, and they probably decided that this was the lightest and worked the best. An extra cam, sprocket, and longer chain would probably have similar if not heavier rotating mass, with more machining required. As we know now, a conventional 4-valve head would have worked better, but they were pushing the envelope at the time, probably without computers to help them design it.
I'm reminded of Yamaha's decision to stubbornly stuck with the 5 valve engine, even when it was no better than 4 valves. I think sometimes manufacturers stick with unconventional solutions for marketing purposes.
However, as far as I know, these RFVC motors have some of the highest specific output for a mass produced, air-cooled single cylinder motor, so they must have been on to something.
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