Astro.
You can make anything 'make sense' if you paint a scenario for it.
It's a damage buff. It's bottom line. Any other sort of justification not only doesn't hold water (A party is going to be well within range of each other for standard 15-20y radius) as well as failing on its own merit (Because it also buffs the Red Mage's spell damage, which can be done at range, and at least one spell likely will; The combo finisher following a displacement)
Why shouldn't Red Mage have a decaying 10% to Damage? It averages out to 6% all damage with 16% uptime (1% RDPS on average) compared to -just Bard's passive crit bonus- which is 2% crit, which at base crit levels is 1% more average damage.