but i think the point of this is to communicate the fact that they're abstracting out the server and scalability details. i'd pay to not have to worry about all that stuff and just work on the app.
if this thing will handle the server for me, keep it secure and up to date, provide me with a database that will scale with my app like amazon's RDS, provide me with meaningful data and metrics about my server/application/database, and provide me with things to make common tasks easier (deploying), then imo, it is worth me paying for it, or paying a small premium on top of what i'd be paying amazon or whoever. but then again, i'm a solo bootstrapper. the more time i have to develop, the better.
"heroku for php" might not be a great choice of a tagline from a strategic perspective, but it is a decent one if you want to consider SEO and organic search results.