The simplicity of it is logic defying. It used to be that you had to find crosswalks or move puzzle pieces or type blurred letters and numbers, but NOW all the sudden I can just click a box and HEY!, I’m human?
That’s hardly the Turing Test I’d expected.
So really, “no value in mixing,” which is distinct from “don’t mix.”
The latter implies a security risk could be created.
The risk of mis-ordering your layers is a security issue.