If Tanks and Healers want to DPS, then embrance it and tie DPS increasing mechanisms into their role.

Have skills or traits that deal more damage when you have more emnity.
Have the a job gauge increase when you heal and be spent on offensive skills.

Maybe the DRKs emnity combo could generate more blackblood.
Maybe WARs Berserk could increase crit rate in line with enmity.
Generate gauge in shield oath and spend it in sword oath.

Lilies generated with healings spells could increase the damage of Holy.
Gravity's damage could be modified based on how many aspected benefics and helios' have been used in the last 30s.