It wouldn't be a balance problem, Paizo just really don't like casters getting to hit things in 2e, so they made sure that polymorph spells leave you less effective than an actual martial and that you can't use magic to mitigate that. "It wouldn't be a balance problem, but here'...
Yeah, it's mostly because sometimes higher level spells have the power to solve whole obstacles/problems/mysteries with a single action, so if the GM wants something to be potentially a substantial undertaking they've got to plan somehow to preclude those spells. The cleric/druid/witch don't ...