I suspect that the restrictions are intentional. I kind of like it the way it is: a wizard's familiar will have an elemental basis (with the other spells listed on the WOTC site) and a cleric/wizard's familiar will be planar.
It also allows the Mystic Wanderer PRC, and other clerics PRCs that gain familiars to have a unique feel to them.
It also allows the Mystic Wanderer PRC, and other clerics PRCs that gain familiars to have a unique feel to them.