At least on Super Mystery Dungeon, a motivated Unown doubles the amount of experience the party gets in a dungeon, and since all Unown count as different Pokemon, the chance of getting a motivated Unown is likely (not sure about some permanent formes) 29 times higher than any other Pokemon.
They still suck anyways.
But an ability similar to Wishiwashi would make sense. The series has pointed out several times that Unown is extremely powerful when on groups.
I remember someone (can't remember who) articulating the point very well, that Unown is treated more like a plot device than a Pokemon. As in, it's shown that Unown have phenomenal power (some speculate that they and their dimension existed even before Arceus), but in battle that power can't emerge.