I usually use middle paste on a desktop mouse but keyboard paste on a touchpad device. As for copying on highlight it depends how you manage your windows, if you use a traditional floating WM and typically click windows to select them you end up with a lot of single character copies messing with your clipboard. Doubly so in the touchpad case again. Or if you like to highlight to bulk delete/replace or if you like to highlight to simply highlight the section on your terminal as you read a manpage or whatever in another window or probably more use cases that didn't immediately come to mind.

Point being it's more about use case matching than one option being the dark ages and another being The One Right Way™. Layer on that some like using clipboard history and others just want a single parking space and it gets even more blurry.