This is one of those use cases that I'd be interested in seeing some research on. Personally, it depends a great deal on what I am doing, but I often use my thumbs to do minor navigations when within a document. Particularly because I have some RSI issues and using my thumb seems to less of an aggravation. Milage, I'm sure, varies, but I suspect that there is a subset of folks that at least use their thumbs a bit.