In a general sense I imagine that spell casting will see the most use when AoE damage is necessary while swordplay will be the primary method of dealing single target damage, although it would be nice to see some interesting interactions between the two. In FFXI there was a Magic Burst mechanic which involved casting a spell for high damage after a successful weaponskill combo was executed. Obviously it wouldn't have the same level of team interaction, but a similar mechanic could be self-contained within Red Mage.