casters can melee attack while casting instant cast spells such as ruin II and bio (I'm not sure about black mage).

apparently you can get free damage 25 damage potency or so every 3 seconds if you right click the target while casting beside it. I'm almost sure that this is not intended as this could be considered another source of damage that is worth putting into the rotation.

if you are a summoner, go and try it, go to a training dummy or a mob and start spamming ruin II while you have the target right clicked at, currently my ruin II is doing 185-190 damage and while spamming it, I see another damage beside it that is hitting 65-68 damage (book attack shazam!).

oh it is also affected by raging strikes, mind you again that this is pretty much free damage source with no mana requirement, the only catch is that you have to be at melee range, which is not that much of a problem imo unless the fight has an anti melee mechanism such as ifrit and garuda. titan will be no problem, or any of the AK bosses.

http://oi41.tinypic.com/23h1v9w.jpg