Clipping and dissecting a coin into smaller pieces for down-conversion are very different things. The piece of eight wasn't haphazardly cut, but instead pre-indented for breaking cleanly.

If you want to buy something worth 6% of a gold coin, whacking off an edge of one is a weird way to do it. You'd need a scale handy.

There had to be other mechanisms, nobody was clipping coins as part of any kind of honest commerce.