While I get the idea, we then go against the balance philosophy of sam and black mage by doing this.
The reason why they're at the top of the scoreboard for rdps is because they don't have any kind of utility whatsoever for the party. No damage buffs, no defensive buffs/debuff apart from role action (feint/addle). Increasing the amount of support these jobs can give would then require a damage nerf to balance them out, otherwise everybody would just play them since they're the strict better choice for your party. While I also like your idea of boosting the party bonus a bit more, let's not forget that the reason this party bonus exists in the first place is to warrant peoples bringing every role in content. Now imagine what would happen if by bringing a black mage instead of a bard you could almost balance the loss of a ranged dps in your party bonus, AND have way more damage than the bard anyway with this job... This would just kill bard in any sort of content.
Balancing skill trees for jobs like these are what makes it tricky to implement a skill tree, not to mention that you also have to give peoples a decent alternative each time, an alternative that is not a trap or completely useless.