From a metagame point of view, is there any reason to take evocation if you can use thaumaturgy at evocation's speed?
Of course there is. Even the most powerful thaumaturgist can only call as much power as his conviction in an exchange - there isn't a way to boost thaumaturgy's power, only its complexity. And thaumaturgy attacks don't have weapon rating - they're just attack rolls.
Now, compare with a feet-in-the-water focused practitioner with Conviction 4, Discipline 4 and a +2 control focus, pretty much the weakest evocation-based caster you can build that's actually focused on his magic. He gets +6 attack roll (the absolute best a thaumaturgist would ever get) and also gets a weapon rating 4. The weakest evocation-user can deal 4 more stress than the strongest thaumaturgy user... and the thaumaturgy user still needs sympathetic links if he wants to directly affect people. As for what a really powerful evocation-focused caster could do in a round... I'm currently playing a pretty strong warlock. She can do Weapon 16, Attack +16 spells as rotes. If she really pushes it, she could level a dozen skyscrapers in one blow or rip apart a small army.
Even the book's example of rushed thaumaturgy takes three rounds after preparation.
Because it still is a strong spell for a rushed thaumaturgy and it takes 3 exchanges to control the power. When we're talking about one-round thaumaturgies, we mean 3-4 shifts for most casters.
Particularly since you avoid taking mental stress for low powered spells.
No you don't. Someone with Conviction 5, Lore 5, Discipline 4 who has a +2 control for a certain type of thaumaturgy will roll control at +6. When casting a 5-shift spell -- and offensively that's an attack roll of 5 with a weapon rating of 0 -- he will fail his control roll 1 time out of 3. So he either takes backlash (stress) or the spell goes to fallout, which can still cause him stress. Only powerful casters could ever cast rushed thaumaturgy without stress... and it would still be inferior to other powers or weapons (or even unarmed attacks) in direct combat.
Perhaps more importantly, how are you setting up the symbolic links that quickly? They're not optional.
Symbolic links are, of course, still needed. But they aren't always necessary. Area illusions and Wardings don't need them because you're (persumably) already touching the area you want to cover. Similarly for worldwalking or shapechanging because you're affecting yourself (and presumably you have access to yourself - if not, seek special care). But almost everything else? Yes, needs a sympathetic link.
All of the above combined means that, while you can do combat thaumaturgies, such types of spells are impractical. A martial artist will be hitting harder in combat than a thaumaturgist, will run no risk to take stress, and doesn't need a sympathetic link to harm the target.
