Howard,
Sure- I used the term "true" and instead should have used "enveloping". I also see that I used helical incorrectly- I meant to say that many open back machines use a spur gear with the tooth cut at an angle rather than a true helical gear. This is what leads to the the point rather than line contact issue. I could be wrong (those things are small!) but I don't think so.
I don't know why others don't advertise the use of an enveloping worm gear. I'm also not sure that other makers don't, in fact, use them. I've never taken a sealed Gotoh apart, or seen a Robson up close. From seeing one picture of Rodgers setup I am assuming they are using a helical gear but I don't know that for sure either.
I'm also not an expert in this stuff! Very interested in it, but no proper training and, so, big gaps in my knowledge.
As for others advertising that feature (or not) I would guess it is because most guitarmakers and players wouldn't know the difference. Graf's site has much more technical data than the other makers' sites, which seem to cater primarily to (classical mostly) guitarists.
In my own thinking of how to make the sets it would be much more complicated for me to make the enveloping gear. I would want to hob it and I would expect the fit between the enveloping worm gear and the worm would need to be much more exact than the fit between a worm and angled spur gear. Making/sourcing the hob and accounting for wear against the produced worms would be critical as well.
I did spend some time a while back looking for stock sets in the sizes needed and did not have much luck. I also inquired to a few places to make me custom hobs and had no luck there either- I imagine it is not worth their time to make one or two here and there. I didn't conduct an exhaustive search, but enough to know it wasn't going to be simple.
And, as you mentioned, does it matter in this application? Since most of us agree that Waverly machines work well I would say not really. I would expect a properly fit enveloping gear to last longer (or at least take longer to develop backlash) and possibly feel smoother at the upper end of the string tension but otherwise be very similar in use to the simpler gear set which most of us are very satisfied with.
I wasn't trying to insinuate it was necessary- rather I was trying to illustrate that the type of gear used is another way to differentiate between the boutique tuning machine makers.