I'm still new to the game, but I'm playing an Emissary of Power who's grandmother is actually a high ranking Sidhe of Summer & she granted him Seelie Magic. I figured that the 4 focus item slots that you get with paying the full price for Sponsored Magic at -4 Refresh would work really great if most of the slots were broken down into Enchanted Item slots; my character didn't make these items, they were all gifts from his faerie grandmother to make sure he was protected. I've got an anklet of speed, faerie-stitched jacket, & Summer's fire ring.
As far as the element, my character has no other spellcasting ability, so I see "Summer" as the only element that I can cast, which does borrow from the others. Gives me a bit of broad usage within a narrowly defined context. Every time I use my magic, it has to match Summer's themes of wildness, birth, growth, renewal, & warmth. Plus, with Summer's magic, you can do some cool biomancy healing effects with the speed of evocation. I would think that the "element" gained by having Sponsored Magic would really have to be related to the Sponsor who gave it.