The problem is with the spell schools themselves being an inconsistent and less useful way to organize spells thematically.
For example, if force-effect spells were all part of the same spell thematic, including Fly, Telekinesis, Unseen Servant and Shield, I wouldnt want the Bard to have it. (But as a Magical Secret it is fine.)
But as-is, the spell schools are a clumsy way to organize spells. The source lists of Primal, Arcane, and Divine, are even worse.
Within this dysfunctional D&D spell school tradition, the Bard might as well have all Abjuration spells.