Ok, I think I get it now. If we gave hygienically-quoted expressions to mappend, they would be in a different closure than the surrounding code and therefore couldn't see the variables it introduced (r, in our case). So I guess you might say that #` should usually be used at the start of a macro, and ` after that. Still confusing.
It's true, it's not quite as intuitive as I'd like it to be: I, too, originally envisioned it entirely replacing normal quasiquoting for macros, but this turned out more difficult than I expected. It might be possible to make it more intuitive, but at the moment I'm not really working on it; this is an after-the-fact explanation more than a design document. I'm presently in the process of creating a low-level systems programming language with Arc (or at least a Lisp) as its metalanguage; when I get the the problem of adding macros to that language I'll probably give this another look.