> I'm firmly convinced the MVP culture of launching as soon as you've built a UI with Bootstrap and thrown it up on Heroku is bonkers

So totally agree with this. All of your growth experiments fail if your product doesn't have the basic stuff right.

It works well though when you are building a tool that lets you do something you cannot otherwise - if that works, even a minimal UI release can get you users. Eg. Stripe (when it released).