"it would never cross my mind to make a blanket statement that all engineers should work at startups" --> I completely agree. In fact, such blanket statements about almost any kind of advice is often naive.
A big part of me believes "you don't need an MBA to be successful in tech" (etc etc etc).
However, every week I use tools & skills that I picked up during my MBA. Yes, sometimes it has to do with modelling and such stereotypical MBA skills. Many other times my MBA has a much larger impact - recalling the themes that were incessantly driven into me in my Managerial Leadership class, or enforcing the rigor that my Marketing prof required of his students when writing positioning statements.
I believe my MBA has increased the likelihood of me being successful. That's what matters to me. I agree that it's not for everyone.
The UX world has struggled to find its best fit in Agile. At SIGCHI Montreal a few years ago, it was a big topic of discussion where pain points were discussed much more than proposed solutions. I like Jeff Patton's view on UX in Agile, which is in the same vein as the 3 points you wrote down.