Thanks for actually taking the time to watch the webinar, I suspect a lot of people wouldn't bother. However, your take on what Devin says is very different to mine, I almost wonder if we where watching the same webinar.
Here are a few very negative quotes which pretty much say turn it off:
"...in some environments, if you turn on MU-MIMO, your environment completely dies right then..." (29:44)
"...almost no chance of giving you value but in a bad environment no chance what so ever..." (38:04)
Here he DOES explicitly state to "turn off" the MU-MIMO on the APs that are capable of this functionality.
"...the first thing I do all of my systems, that I implement, is turn off transmit beamforming and turn of MU-MIMO support. If the vendor's equipment supports that, I turn them off immediately. They are very high overhead and give no meaning value what so ever." (44:38)
Hope this helps.