I think the number of times I use 'apply with a variable in the final position is enough for the syntax to be relevant to me... but then I'd be likely to refactor (a b c . d) into (apply a b c (something-else)) and vice versa all the time, which would be a bit of a pain. Then again, it's balanced against the cost of refactoring (a b c) into (a b c . d) and vice versa....
by the time the compiler sees the expression there's nothing there to tell it that a dot was used.
In Racket's syntax there is a way, I think. It makes syntax more complicated than just its conses, though. You'd have to use an Arc variant of syntax-quasiquote (or just syntax-quasiquote) to construct it in macros.
Meanwhile, the (cdr `(foo . ,<scheme-expr>)) quirk would become more of a bug than it already is. But then I assume whatever Arc hack implements this syntax would also have '$ built in, so there's probably no point to keeping the quirk around.