I think the number of connections is more than enough, keep in mind that its concurrent connections. If you're using so many connections where their price seems like a sticking point you should probably roll your own.
It's not too hard to roll your own, I wrote my own over the course of a week in 2007 using Flash XMLSockets and Twisted Python as the backend, check here on hints if you need ideas on implementing your own: http://code.google.com/p/aonic/wiki/Examples
To be fair, "more than enough" depends on the problem being addressed. It's trivial to have tens of thousands of concurrent connections on a $20/month VPS. You get 100 concurrent connections on PusherApp for $20/month. That's a huge difference. Sure, it just works and you do no administration, etc. PusherApp seems like a convenience service for startups with money to burn.