You should really read the thread and post I linked above, Patrick goes over exactly what they're getting out of the new arrangement.

Also, Joel's stopped blogging for the most part so Joel on Software links might not maintain their current worth forever.

I'm not sure it's possible to have "enough" inbound links, assuming you are able to make a profit on each new customer. Given a certain level of profitable business and submaximal SEO, effort aimed at improving SEO and conversions will be the highest bang for your buck.