The one possibility I can imagine is if they eventually decide to make the Psionic source. By doing power groups they'll be able to at some point (like if/when they do Dark Sun and decide at that point to introduce the Psion) demarcate all the spells that are considered "psionic"... because these will be both spells that are currently in the game, plus probably new spells they would create for the source. By putting something like Telekinesis into the Psionic power source (in addition to it already being in Arcane)... it perhaps helps make it feel to some people like it is its own Psionic power, rather than just a spell "borrowed" from Wizards.
And then by creating the giant pool of what are considered psionic spells... they could also start divvying them up amongst the Psions's subclasses (if they so choose) based upon their schools. So the Telepathy subclass would get Divination psionic spells plus maybe a couple other school options... the Psychometabolism subclass could get Transmutation psionic spells and maybe a couple other schools... and so forth.
Granted, I'm just spitballing here... but by creating power sources for spells you gain a bit of stylistic identification. These are Arcane spells... these are Divine prayers... these are Primal evocation... and these are Psionic Powers. Yes, there will be some overlap with some being in more than one group... but for the most part they might help solidify spell identity.
When updating the Spell Schools
to tag spells more thematically, it is easier to distinguish between:
telekinesis, force constructs, magical energy
scrying, fate, teleportation, planar effects
elemental effects, earth, water, air, and fire
planar darkside, Undead, Fiend, Aberration
life, lifeform, body, shapeshifting, healing, plant and animal
While subclasses might do differently, the base class generally feels like:
= Conjuration, Evocation, Illusion
= Conjuration, Divination, Enchantment, Transmutation
= Divination, Enchantment, Illusion, Transmutation
= Divination, Evocation, Transmutation
= Divination, Necromancy, Transmutation
= Conjuration, Divination, Illusion, Necromancy
= Evocation, Necromancy, Transmutation
Instead of the amalgamated UA lists of Arcane, Divine, and Primal, the class can refer to the schools. Then designers can modify each school when adding new features that several classes can share.