There is a specific character concept that is more powerful than a lot of others: Wizards. If all anyone cared about was power, then everyone would play Wizards with maxed out magical stats. Some people do base their character decisions on the theme and character concept rather than cramming in all the bonuses they can get away with, after all.
And aren't sponsored magics supposed to have benefits over the standard powers? That's the whole appeal of making a deal with the devil for Hellfire, after all, it gives you more power at less cost than taking Evocation and Thaumaturgy, with the caveat that there's an agenda and debt that come with it. Sure, taking Rune Magic makes you better than a normal ritual crafter with two Refinements. But that ritual crafter doesn't have Odin looking over her shoulder, giving her inconvenient orders and potentially taking away that power if she steps too far out of line.
And I don't see how what I said enables "incoherent concepts," since I meant it the opposite way: The character concept should define the powers, not the other way around. A player shouldn't cherry pick powers and then cram in the concepts to justify it, and a GM ought to step in and say something if he sees that sort of thing happening.
That said, I just noticed the Rune Magic write-up doesn't mention the standard Sponsorship benefits, or is that just not mentioned because, well, it's standard?
And anyway, like I said, while I'd prefer 6, I can live with 5, so if it's that big a deal, go ahead and write it up that way.