That would be really nice indeed.
Note: in XBMC there is, however, already a pretty good script available for downloading subtitles (OpenSubtitles OSD). In Plex/Nine (which is what I'm using) it's currently missing (was working in Plex/Eight) but will be implemented in a later stage. You could therefore argue that implementing it would be doing "double work".
I am aware. MediaPortal has similar features, atleast in the My Tvseries plugin. However the MediaPortal implementation is based only on filenames and not on movie-hashes, making it a bit dificult to determine the correct subtitle when in the GUI. I am not aware how XBMC handles this.
It could be considered double work, but I would argue that the work should actually be done in the backend downloader, and not in the GUI MediaPlayer. CP (and sickbeard for tv-series) already handles downloading of the video files - it would make sense to also let it handle downloading of extra files like trailers (which it already does), subtitles and perhaps even Fan-Art and covers.
The reason for this is that CP is running 24/7 (for most of us anyway). This means that the extra files would be instantly downloaded when available. If the downloading is handled in the GUI, they will be downloaded only after the GUI is started - at which point they might (in theory atleast) be too late since at this point you actually need the files.
I like all my stuff downloaded automatically, for increased Couchyness (can this become a word, please) and WAF. And in my opinion the best thing is a backend solution that handles the downloading of all of these things.