Ok, let's get specific. I've got a pair of Miras, dimensions 112-79-102, and my straight skins are 65mm wide. I used them because I had nothing else. The skins were originally for a pair of skinny touring skis, something like 80-68-75. Using these skins on my Miras I noticed that, even in soft snow, if I weight the edge of the ski more I tend to slip. The effect is way greater than with the skinnier skis. I'd argue that wider skins cost only a tiny bit more, it's really easy to trim them, and I want to go up with skins, so why not get the extra grip, even if you don't notice often? More skin=more grip, period, even if it isn't always noticeable. YMMV.