To my ears rolling off highs on the amp seems to affect highs that aren't affected by rolling off the tone on the guitar, and different frequencies are affected by gauge and brand of string, as well as speaker and pickup choice. Yes, there is some overlap, but not all high frequencies are affected equally or in some cases, at all. That's why I use these different tools in different ways for different reasons. If the tone knob on the amp or guitar were enough by themselves, then I'd never have needed to work with pickups, strings, speakers, pickup height to get things dialed in. Playing a telecaster is a never ending battle with high frequencies, at least for me.