As opposed to simply writing something along the lines of, 'The cost of Sponsored Magic is reduced by one for each of Evocation and/or Thaumaturgy also possessed.'
See how that's shorter, clearer, drops all of that extraneous talk about 'tacking things on', has absolutely the precise effect on the rules as your current interpretation, and so entirely bypasses this controversy?
There's a serious issue of writing style to consider here. I love the DFRPG and it's authors and it's writing style to an extent that may not be entirely healthy, but, well, the sentence you stipulate would fit in about as well with the style the book's written in as one in a foreign language. And wouldn't say anything about you gaining
any benefits of Sponsored Magic if you already had Evocation and Thaumaturgy, which would potentially start an entirely different argument.
But no, that's not what is said, regardless of how you've decided to interpret it.
Again, this assertion that your interpretation is fact. That's not remotely proven, and indeed you've presented zero real evidence of it.
The source gets tacked on to existing spellcasting Powers. It's not the Power that gets tacked on.
Okay, for the sake of argument let's assume this is true and explore the implications of that. The rest of this post makes this assumption (which, ftr, I don't necessarily agree with).
In Sponsored Magic without Evocation and/or Thaumaturgy, "the sponsored source of power replacing the usual specialized focus", ie. it takes the place of evocation elements and thaumaturgy groupings.
When used on its own, yes. But I thought we just established that the source was tacked onto Evocation and/or Thaumaturgy, not the power per se? It's the power that lets you use the source in that manner...and I thought only the source was transferred?
Now add Evocation and/or Thaumaturgy back in.
Okay. This is the bit where that whole 'the power' bit gets taken out.
The source - going back to the previous sentence, that is 'the sponsored source of power, which replaces the usual specialized focus' - gets 'tacked on' to the existing spellcasting powers - that's Evocation and/or Thaumaturgy.
But that first part doesn't apply, since it's part of the Power that's not explicitly being tacked on. Right?
Let's just deal with Evocation + Sponsored Magic for a moment.
The sponsored-source-element-equivalent is now 'tacked on' to Evocation. Evocation can have Specializations purchased for its various elements. What might something serving as an element and 'tacked on' to Evocation mean in terms of Specializations being purchased?
It
might mean what you say it means...also, it might not. Also, there might be the specialized rules Dracorex mentions that explicitly deal with this situation...
Again, in the absence of clearer language, additional language, or the removal of extraneous statements, I can only interpret these passages as allowing the purchase of Specializations for Sponsored Magic in the presence of Evocation and/or Thaumaturgy.
And yet you appear to be the only one with that limitation, as I can easily interpret them the other way, and most people appear able to interpret them
either way, since they find it to be unclear...