Did you not read to the section about jRebel/Play and not needing to restart?
But I do agree with you that server startup time isn't as big a deal if you don't have to restart after each change.
Businesses tend to be wary of building long-term solutions in flavor-of-the-week frameworks/environments/etc. Even when it's just a dev tool, even assuming that the tool behaves exactly like a real production environment, there's overhead to throwing away your workflow that Just Works and getting everyone used to something new. And developer culture right now is that there's a new must-have "Hottest Tool/Framework/Library That Will PUMP UP YOUR PRODUCTIVITY!" every couple months.
And rightly or wrongly those tools are often a hard sell to management when there's ongoing costs - we can't get funding for a JIRA server let alone JRebel. Yes, the time it saves me would be worth more than $1.50 per work-day, but that's often not how managers look at it.
A long start up time makes more difficult your integration/functional tests that it should.
And, AFAIK, Play! is also fully debuggable. What's bothering you?