1. You can already get the bonus by having relic hat on.
2. It's nice to have some extra reason to invite RNG or DRK for better rolls. Or else why invite DRK over a MNK if both provides same lv of rolls?
3. The biggest game changing song is march, and maybe scherzo if you insist. However, giving 3 rolls or 100% job bonus doesn't make COR as strong as BRD as a buff job, unless there's haste roll.
4. I don't want 3 rolls either, this job is supposed to be a buff/DD hybrid job, spend more time rolling = less DD. Ppl interested in full time support can play BRD.
5. I've cleared all delve2, skirmish 3, all AA and other battle field like DM2/Celestial nexus on COR and it worked just fine: Not a square peg into a round hole. It's not a "required job" like BRD WHM, but it has more use than PLD RNG and MNK. Ppl with just MNK leveled still need to lv RNG PLD etc to do marj delve, and vice versa, ppl with just RNG leveled can't do yorcia. Or else they'd become a leech.
Is it as useful as BRD? No. But it's not like every other job is as useful as BRD anyways. Personally, I prefer it in most events except yorcia delve2.