I've had three runecasters and a true bow, and the thing is, they can overpower any encounter in just a two turns, so those rites aren't really going to be necessary. Two Blasts can wreak havoc, three is total cheese. Four? Thermonuclear.
indeed. the invocation will at best increase one character's attack by 3, or a 21% increase in chance to critical hit. much more efficient to spam attacks. still not a bad thing, since its a minor ability
I guess a four bonus would be the maximum (four runecasters focusing their rites on one of them), giving a maximum attack number of 20 (Lvl6+Wp10+Rite4). That's going to mean a Fireblast with a high chance of getting all criticals, but fireblasts have a pretty good chance to get lots of criticals with just the 16.