USRSA has recently run out of these H-adapters, and Prince hasn't had them for quite some time. In concept, they're a good idea, but in practice you are asking two pieces of plastic to negate the frame flex induced by hundreds of pounds of main string tension. It would be interesting to measure the frame deflection with and without the H-adapters, but I'd bet the reduction in flex is just about nil.
The new Silent Partner machines come with W mounts at 6 and 12, which work sort of the same as the H-adapters, but like the H-adapters they crack after awhile. This is not a knock on Silent Partner or the H-adapters, but rather a realization that a piece of plastic is not going to be able to withstand this type of abuse for long. I have also seen some metal H-adapters, but after they bend, they're not supportive any longer.
If this issue really concerns anyone, I would direct his attention to the Prince line of stringing machines, which offer two widely-spaced supports at 6 and 12, which are designed to do what the H-adapters can only dream of. I believe there are other machines with similar mounting systems, but they're not coming to mind right now.