I think that would definitely be a useful feature. If it is technically possible I don’t really see why this shouldn’t be adopted. With this, it should also be possible to change the punctuation between title and subtitle, right? If yes, that would solve the “colons vs. dot”-problem that scholar writing in languages other than English sometimes have.
While I cannot speak for @John_MacFarlane, I also have the impression that this might be useful for pandoc-citeproc. Right now, pandoc-citeproc merges biblatex’s title and subtitle into CSL JSON title using hard coded delimiters, which could be avoided if we had corresponding variables in CSL. (Of course, biblatex’s titleaddon will still need individual treatment, but having distinct fields for title and subtitle would be a good start.)