With those rules it's all but completely useless and as a player I'd never in a million years use it.
That mental stress would be much, much better used for evocations during combat.
At what point do you stop nerfing abilities? If you nerf them too much, they aren't worth much anymore.
Give me one example of a wizard using his magic in this way to begin with in the books... And ANYWAY, you can only use certain fields of thaumaturgy at the speed of evocation, depending on your sponsor. Summer's thing is Biomancy, Winter's thing is Entropomancy... For what your using it, stick with Evocation... Besides a couple of quick benefits, like using summer to help mend a friends wounds RIGHT NOW, or placing an Entropy curse on your opponent to put him at a disadvantage or something else, depending on your Sponsor's field, if it even has one... Its just one thing a Sponsor can give, they have a lot of other benefits...
And it even states it earlier in the books as well as the RPG.. Thaumaturgy really isn't ment for combat, but a clever player/character could use it to give himself an edge, in a good number of situations... Its just really, the nature of the beast...
And I don't have a problem with using the RAW... I just don't like a player looking at them, and either doctoring, or cherry picking because of poor wording, and trying to get the best benefits they can manage...