The benefit would be to reduce or eliminate valve float at very high RPM, especially with a high-lift cam. In engines continously running at or above 7000 RPM, there is very little time for the valve to fully close...the extra springs will help snap the valves fully closed.....and two springs are less likely than one to fall into a "resonance" at certain RPMs (this can cause the springs to "freeze" somewhat).
For a normal street car, they might actually be a bad idea since they slightly increase friction and may add to cam wear.
I'm running my mostly-stock racer with single valve springs and I limit myself to 6000 RPM. It's not a problem in my application. If I had a hotter engine that could benefit from the increased RPM, I'd consider them (there's also a point to be made that these 1500 engines only have only three main bearings and aren'r really suited for +6000 RPM).