Although I find the extra hole concept appealing,the reality may be an instrument that is more dificult to play instead of easier.
Based on my experience with an 8-hole “modal” Overton, which has an F-nat thumb hole and a (low) C-nat hole for the little finger of the left hand, there’s no doubt that it’s more complicated than a standard whistle.
I use this whistle for dance gigs and sessions in very noisy pubs, but 99% of the time it might as well be a standard whistle, the way I use it.
After some eager experimentation at first, during which I discovered that tunes such as The Bunch of Keys, Splendid Isolation, etc. were suddenly much accessible, I haven’t put the requisite practice into really mastering the animal. What I find most difficult is the “pincer” action involving both thumb and fingers to move from F-nat to low D (or worse, low C). You also have to work out when to put your thumb back on its hole when moving from the F-nat up to higher notes involving the left hand only. It just needs serious practising. (It’s on the to-do list!)
The possibilities are real, though. I’m sure Mary Bergin, with her amazing half-holing technique, and other players too, could play you a tune such as the Bunch of Keys etc. with great aplomb. But it’s not a tune that whistlers seem to flock to. And to me there’s no doubt that you can do the job more easily and more cleanly with a thumb hole. You can also get a decent roll on the thumb hole, which I’d love to hear someone do with half-holing.
As for Peter’s point about colour, well you can still use half-holing if you want to.
I’d never have thought of the modal design had Colin Goldie not suggested it to me. But if you accept these innovations, it seems to me there’s no reason for not adding a G# thumb hole for the top hand. Which would definitely complicate the playing of the instrument in passages that involved removing both thumbs in quick succession. And if you accept a G# hole, well, why not a few keys here and there… you’ll quickly end up with
a curiosity.
I’d be surprised if extra-hole designs become widespread, let alone standard. (I’m only interested in IR-Trad, as most of you know. If I wanted to play church music I think I’d go for the saxophone, not the whistle.) The main reason is that not many people will want to learn technique that is of no use on 99.9999999% of the whistles circulating in the world. (That’s why everyone sticks with the standard typing keyboard, rather than learn the apparently much faster more efficient and less straining Dvorak layout.)
People come up with what they claim to be improvements on the design of the violin with amazing regularity. None of them every catch on, though…