The entire promise of the space is that the customer only has to worry about his own code and perhaps tweaking a few knobs.
Rails is very widely used. How can you consider that an edge case?
Since even many Rails apps now do not follow a single threaded request-response model, that model of running a web application needs to be considered as one case of many, and building a platform that supports many/all use-cases as well as possible is more complicated than building a platform that fits one use case like a leather glove.
I think statements like these obscure away the very tight coupling Heroku has historically had with Rails. While certainly Heroku now perhaps envisions itself as a do-it-all PaaS, there's no denying Rails at one point (and, numerically, perhaps still) was their bread and butter.
While I don't have numbers to support or refute the assertion that "most Rails apps are primarily single threaded", my suspicion is that this is in fact still the case.
I'm taking this example out specifically, because it illustrates my point quite nicely.
If there were a sizeable community of people who only wanted to use a computer for Photoshop, and tailoring the windowing system to them made it a significant usability improvement for those people, then it would be a completely imaginable situation that upon first opening your brand new Mac, it'd ask you whether you're one of those Photoshop people and want the special windowing system setup.
Well, ok, haha Apple and customizing anything for anyone, ever. But many other vendors might make such a choice.
The apparent stubborn refusal of many PaaS services, including Heroku and yours, to particularly tailor to a very common configuration of Rails sounds like a hole in the market, to me. As a customer, I don't care whether this is "incorrect" because Rails does not conform to some yet-to-be-defined standard or simply because the PaaS doesn't have their shit together. The customer experience is the same: I'm running a blog tutorial app, and the performance sucks.
And Rails 4 is going to bake-in "live streaming", making single-threaded app servers even more of an edge case.