Most film doesn't benefit from scanning over 4000 ppi, but some films with very sharp lenses and images shot with impeccable technique can benefit from higher resolutions. Unfortunately, more pixels doesn't always equal higher resolution. There are only two scanners I'm aware of that can actually come close to 8000 ppi in the scan - an ICG and a Howtek/Aztek 8000, both of which use a 3micron aperture to determine the resolution. The Heidelberg drum scanners, which were designed for prepress and not necessarily for fine art, use somewhere around a 10 micron aperture, which effectively limits the maximum resolution, regardless of what you think you're getting.
In addition, drum scanners, when you're at the upper end of the resolution ladder, generally jump - and this is for the Howtek for sure - from 8000 ppi down to the next step at 4000 ppi, then down to 2667 and then to 2000. ICG's number will be slightly different depending on the specific apertures they use, but the principle is the same. When the scanner "sees" the film through a hole that is 3.175 microns in diameter effectively - well, multiply that by 8000 and you arrive at 25.4 mm. That's how you get your 8000 ppi. Any drum scanner that uses a larger aperture can make finer increments in the stepper motor to get the steps in one direction, but is still interpolating in the other. That's why the Heidelbergs max out around 5000 ppi in real world tests and the Howtek/Aztek can hit the mid 7000's.
Now, the films that can actually take advantage of that are things like T-Max 100, Kodachrome 25, Technical Pan and maybe a well shot Velvia 50, and even then only when shot under optimum conditions. For the rest 4000 is more than enough and for color negs, generally far less than that. But that's a whole 'nuther discussion. And we haven't even talked about tonality at all yet.
