Preface: I’m coming from a PvP standpoint.
I used to try to dismiss the notion that Thaumaturge was a lackluster class, and still, after discovering that Swell and Shrink Body are melee spells, I don’t think that the class is bad as a support – however, this knowledge is a definite game changer for me.
I can only see Swell Body’s slowing attribute being kinda useful against melee because, well, they’re melee and have to run up to you. Still, you’re putting yourself in so much danger versus the +60% damage boost that Swell Body brings only to slow someone for a couple of seconds. AND that’s if you’re even near the melee player. What if they’re off chasing another player on your team?
Furthermore, I can’t imagine any ranged class letting you walk up to them to shrink them for -75% damage attribute. 
Has anyone had experience as, with, or versus another Thaumaturge in PvP and how useful their Swell/Shrink bodies are versus other players, be it on your or the opposing team? I’m almost hoping that someone says that they’re both still useful in PvP… 
Edit: Does the class pick become any more viable if I also have Cryo/Chrono as one of my classes? Meaning, will the melee part of Shrink/Swell Body not be so cumbersome because I can freeze/slow beforehand?