Hej there,
in about two months of training with XERT I think I exclusively got prescribed workouts with XSS/hour. As far as I understand the big advantage of training with MPA next to the analytical insight is modeling workouts to really get to a certain degree of fatigue and also giving a much more individual recovery window.
With XSS/h power will drop the longer the interval goes, but it never brings me to a certain degree of fatigue. Recovery is also given via XSS/hour, so not even that is really with a certain point on my MPA curve in mind.
In episode 4 of the podcast at ~11min it is said that SMART intervals could/should be modeled with the goal of a certain MPA instead of fixed time/power - but they rarely do.
So my question aims to get a deeper understanding is simply: why is that? 
Thanks and cheers from germany
Daniel