So, using your math, if I listen to Pandora roughly 17 hours in a month (less than an hour a day), my entire 1GB high speed data allowance would be used up, and my other apps would crawl. Now, it doesn't matter. I definitely don't mind being able to save $10 a month (which I might have to do if this didn't become reality).
Let me also point out that you are harping on Pandora as if it is the only streaming service T-Mobile is supporting. Just his morning during my trip to the gym, in the gym, and then my trip to work, I consumed roughly 100 MB of streaming music over about 1.5 hours of listening, using Spotify and iHeartRadio. None of it counted against my T-mobile high speed data cap.
Yes, Pandora streams in fairly low quality - both for their own bottom line and for the sake of their user's data charges. But to assume that all streaming music is Pandora is a mistake. Even though Pandora absolutely dominates the streaming market, it still accounts for less than a third of total streaming music.
Most streaming apps are 128 kb, a few being 256. None are 384. Only 384 streaming app I can think of is "my stored" music on Google Play.
I agree, the true beauty is it doesn't count towards your data cap. Most streaming apps will work throttled, but not going against your data cap is where it helps the most.
Now I wonder if Speedtest counts against my data cap.....