Shroud of Saints is on a 2 minute CD. If you're popping that on CD whilst having 'decent' mana management with the spells you're using, you'll almost never go OOM. Chuck in the fact that with raiding you'll always have a Bard and bam, you'll never have Mana problems.
Therefore it doesn't matter if SoS doesn't scale with gear because we can rely on other things (eg. Bard song) for mana. If we didn't, then no class would need that song making yet another song of Bards useless.