Hi Shakespeare47 - I wonder if your T is perhaps just not a good fit for you? There are T's out there that actually CAN explain a bit more about the research/validity that goes into treatment. I had one previously that was involved in the local universities (teaching and also doing research), so he was very on top of that sort of thing. (He was a phd level, I'd guess - but am not sure - that those at the master's level don't focus on research, and maybe that's some of what you're running into?)

By the way, test "validity" actually means something... you might want to google around, or look at a "research methods" text book. I believe (iirc) that it means that test itself has been tested for a few different things... things like: does it actually measure what we think it measures.

Here are a couple pages that talk about it at a high level...
Validity and Validation - Psychology Terms
Test validity - Wikipedia, the free encyclopedia

I think the issue is that most Ts don't care about the research side of things... that's just not their interest/speciality. They may be using techniques that they learned (which were researched by the people who developed them) - or not - but if they're not really mentally oriented towards research, they may trouble discussing what research supports a particular intervention....

Just a thought... it's first thing in the morning for me and I didn't sleep well, so hope that's helpful and make sense... feel free to ignore if it doesn't!