RDM in endgame

[Jakuk]

They have 100% uptime. Why does duration matter?

Because they need to recast every 1:30 whereas this spell is 18+ minutes, it's also an additional effect on another spell, not a spell that does just this buff. Besides I don't think the numbers are low anyway, maybe defense but then it still gives only 25 less than Protect V, which it'll stack with on top of RDM having Phalanx (and if added the way you wanted Phalanx II)

Buffs need to be balanced around what a job gets already and RDM gets quite a bit already.