In general, a melee class primary with a magic class secondary, or vice versa, tends to be a bad idea... simply because the mix usually requires significant statting in too many areas, for it to be an endgame worthy spec. For example, what's a rogue or a warrior gonna need with intelligence or magic attack? Or what's a mage or a priest gonna do with strength or physical attack? But, because your shared abilities dabble in the pool of both types of classes, you end up needing to put some stats in that area to unlock your character's true potential... or you wind up doing what most do, and just "ignore" those skills and abilities. Either way, you end up with a gimped aspect about that character that becomes a significant hindrance.
If there was a way that some of these classes could benefit by a conversion factor of certain stats, they probably wouldn't end up being so unused or useless. For example, if a knight/priest had a magical attack bonus for a given amount of strength, holy light's fury or rising tide might be useful.