True. BUT. I disagree in the case of soulfire, because it doesn't give a specific theme of thaumaturgy, but the POWER thaumaturgy (I mean other sponsors give just the power [-2]Ritual with an appropriate theme, but soulfire gives the power [-3]Thaumaturgy).
No it doesn't. It gives you all the benefits of Thaumaturgy as long as it fits the
Theme and Agenda of the Sponsor.
So when your Fairy Friend (let's say another PC) asks you to help them hunt down an enemy they want to avenge, maybe because you owe them a favour, You might not have access to your Soul-Fire because Soul-fire isn't really about vengeance and it doesn't care about hunting down Fae in the Nevernever.
You'd be much better off with Thaumaturgy.
Dr. Dante Gheil has soulfire, and has discovered a rare infection in the blood of his werewolf patient, the product of archaiozoic bacteriae from the NeverNever. These bacteriae produce a neurotoxin that assaults the synapses responsible for controlling his shapeshifting powers and, by extend, his bloodlust. The Great and Benevolent vultur decrees that to purge the toxin and the bacteriae from his system, Dante needs a 12-shift ritual. With his specialisation in Biomancy at +6, he reaches the required complexity just enough. If he were to use a soulfire themed ritual, though, his base complexity would be +6, not having any specialisation, and he would need 3 successfull declarations (or inflict 3 mild consequences on his patient).
Correct. Because you put all your Foci in to Biomancy and not Soulfire. In this situation, if you'd put all your foci in to Soul-fire, I'd allow the exact same spell because Curing people of Lycanthropy seems like something that suits the Agenda.
If you were wanted to TURN people into Lycanthropes, your foci are better spent on Biomancy because Soulfire wouldn't let you turn people into were-wolves..
You see. While Soul-fire does every type of Thaumaturgy, it doesn't do every type of spell. Why? Because it has an agenda.
Standard sponsored magic benefits
(page 288), with a potentially gentler agenda
(though this may come with a tighter credit limit
on the matter of debt).
and
thaumaturgic application…well,
we don’t know really, so for our current purposes
consider it to provide the full range of thaumaturgy
spells (those which are agenda-compatible,
at any rate).
Notice what I bolded. It DOESN'T give Thaumaturgy. They specifically say they don't know and that you should assume the full range as long as they are "agenda compatible".
This is VERY important. It's not a free, flexible Thaumaturgy with extra benefits and a GM should make sure they recognize this.
EDIT: you pay extra because of the extra flexibility of Soul-fire and because it down-grades ALL toughness, regardless of Catch. It doesn't cost more because they assume it gives you Thaumaturgy.