The consensus is (right now at least) you're never going to really need more than 10-12 turns on the hard bosses. Any more and you might have squeaked through, but probably lost mastery due to actions plus damage taken from a long fight. This decreases the value of r5 spells, unless you're splurging them on trash (and even then there's BM RM2)
At the same time, if you have the rods to support it, magic is very, very clearly still superior to physical damage barring fights that promote Retaliate/Taunt or Retaliate/Advance (like Knight Path Tifa event). Two mages with r4 of a vulnerability element are hands down the best bang for the buck on both damage per action and sustained damage, often getting into the 9k range (as opposed to a r2 -aja that hits for 9999 4 times)
Hence, all that put together, along with the absurd honing costs to get to r5, we arrive at the current 'best practice' being having 2x r4 of each element. It's the most cost-effective, both in terms of orbs, and in terms of usefulness in a limited number of ability slots.
If you've been extremely lucky and pulling 5* swords left and right, then it's probably not as imperative to get two of each since spellstrikes generally work 'just as good' with the 9999 cap and our early ATK soft cap change, but with the caveat that you need to slot in Armor Breakdown into your lineup.
edit: To add to that, the hardest fights are single-battle, where you're ditching Mana Spring II for pure damage RMs like Attunement II, Devotion or Rod Mastery, again closing the gap between -aga and 9999-capped -aja spells on vulnerable bosses.