When our feet land on the ground, we land along the outer side of the foot. This is a supinated position. The degree of this supination varies according to whether we land on the heel or fore-foot, type of arch, lower limb alignment and also pelvic alignment. From that point we pronate from this position. The degree of pronation is also dependent on many factors. For example, a high rigid arch (or supinated foot) will not pronate much or not at all. A flexible high arch will pronate. A flat arch will pronate or over-pronate.
D2D states that he has "virtually flat feet" and that he supinates. I do not know how he comes to this conclusion, or if somebody told him this. Interestingly in an earlier thread D2D mentioned that he wears neutral running shoes. I therefore suspect that he has a flexible arch. However what is categoric is that apart from the initial phase of supination on heel or foot strike, we cannot supinate from a flat arch position. It would have to lift itself off the ground against gravity and inertia.
If you've got a high arch then you will always supinate, same as a low or flat arch will always pronate a bit. Its actually much more complex than that really because of lower limb alignment, pelvis, etc. The important thing is to be running happy in your shoes and injury free. If not, something isn't right and the way we run / shoes are the most common reasons.
I think that the way to look at it is to try running barefoot and see what happens. Most people would automatically land towards their mid to forefoot because otherwise you'd get very sore heels if you landed on them! There is still a degree of supination or pronation at the forefoot so if you have particularly flat arches then you'd still benefit from a degree of control. The Newtons are just designed to try and mimick this by encouraging you to land away from the heel rather than on it. That's not to say that you can't heel strike in them because I've seen people at races wearing them and heel striking. These guys have wasted alot of money!
I've worn the All-weather versions and I've had no problems with grip or traction. That includes doing last years Beachy Head marathon in them and they had good grip thro' mud, slippery chalk and grass. Only problem I find with them is trying to get hold of them in the UK!