My ultimate goal is a way to hilight ALL user-input with some background color. In my mind, the idea that shells should hilight the PROMPT, but not the USER INPUT, is entirely wrong.
Why would you hilight user input? Because it makes it MUCH easier for people standing behind your back, reading your screen, to see what's going on. (or, if you're teaching, to hilight your typing on the projector)
And it's not just me — HTML includes the <kbd> tag that works the same way. Many guides suggest applying some unique styling to the tag.