I would much rather see them tackle this from a party synergy point of view. Two points:
#1 Incaps should be spread out appropriately. At this time it seems OK, but could probably be improved a great deal. Mobs should require incap to some degree to deal increased amounts of damage. Take a battle like Adamantoise for example. Begins the battle in its shell, and you need to incap the shell for it pop out to deal damage, but then it retreats back in the shell. Things like that which have been mentioned in great detail about the forums already.
#2 If each class had some sort of party-wide buff they could activate, and added significance to the party's ability to handle situations, people would certainly think about adding classes more often.
Here are some examples with jobs, names and effects:
WAR - Battlecry - All party members receive "Attack +5%"
MNK - Mind's Eye - All party members receive "Double Attack +5%"
DRG - Exploitation - All party members receive "Critical Hit Rate +3%" when executing combos.
PLD - Righteous Aura - All party members receive "Physical Damage taken -5%"
WHM - Blessing of Menphina - All party members receive "Healing received +5%"
BLM - Elemental Fury - All party members receive "Elemental Damage +3%"
BRD - Rallying Call - All party members receive "Block, Parry, Evasion rates +10%" (bard should have a few to pick from outside of songs)
Would be cool if these buffs also somehow involved the deities somehow (ie. Aura of Halone, Cry of Halone, etc.) Each raid-wide buff provide the party with something that is the strength of that class. They would have to be assigned well to ensure that the buffs are worth inviting that party members. Of course the classes themselves need to be balanced to a degree and the buff should be the extra icing on an already sweet cake. SE could even do something creative link strength the buff in a light/full party, etc.
Basically like what Currahee has posted above.