I don't understand what sanctaphrax means by when he says thaumaturgy blocks are less convenient.
You need a threshold for a ward. You can't just cast a Harry-style shield spell using the Thaumaturgy time-scale.
I still don't understand why thaumaturgical attacks don't have weapon ratings...
No special reason, as far as I know. They simply don't.
Check out Sells' killing spell. Accuracy 36, weapon 0.
And that's not just a weird example, it's consistent with the rules elsewhere. You cast an X-shift attack-type spell, you get X accuracy and no weapon rating.
PS: Given the fairly open-ended nature of Thaumaturgy a GM could allow weapon ratings on rituals or convenient ritual blocks without actually contradicting the rules, but that sort of thing is in house rules territory and not really possible to discuss here.