(Sidenote: subsonic does read your tags, but only to update your search index. )
This is true, and if Subsonic could be coerced into using MySQL it would be even less of a problem!
Radio automation software such as RadioDJ uses MySQL, it doesn't take any time at all to query the database even from an external web site (such as for a request system) so having ID3 tags in the DB is a definite advantage.
Yeah, I know RadioDJ serves a (very good) but different purpose before anyone says anything!