I think it's mostly a color thing. Like you said, other than a few relatively minor things, sponsored magic is mostly channeling + ritual with a paint job. But it can make a big difference in how things play out and how things feel, if the magic is yours or if it's only been borrowed to you.
Having a sponsor linked to your magic will also make you very aware of how your character is linked to that sponsor. After all, you could always draw on more magic, it is just a compel away. And that's future me's problem. It can also be a justification for other powers. If you are already sponsored by a powerful being, you are kind of her emissary, so adding on a few powers after that is less of a stretch than it would be if you had channeling+ritual by yourself.
Sometimes, sponsored magic can come with other perks as well, it isn't limited to the way the ones in the book are written. Intellectus comes to mind, for example. If you take it as an upgrade to your standard wizard powers, there can be all kinds of fun stuff.
I've proposed a while ago, that temporary could just as well be a limitation on sponsored magic as a location would be. In that regard, you could spend a fate point to temporarily tap into the ambient energies of a place for all kinds of cool effects. Tap into a thunderstorm. Or the energy of a gospel choir in an old church. Or the celebrations in a football stadium. You'd have to deal with the sponsor debt a bit differently, since it isn't as though you can pay anyone back, but the sponsor debt can be internalized. You could have flashbacks to the event, causing you to fumble. Or you could have bouts of euphoria that lead you to do crazy and dangerous things. Anything like that, really.
Also, sponsor debt should not be underestimated. It can be the difference between getting that last spell in or not. Evothaum adds a bit more versatility to your spells, which can be nice, but I'm pretty sure more often than not, you can get around it by being creative with your evocations.
So there's a lot you can get out of it, I think, if you look at it as more than just a bunch of numbers.