Regrettably, there are not many plugin developers that subscribe to the model that these guys do. Here's the deal - a fully-functional, feature-packed, rock-solid plugin that's immediately useful and gets the job done - for free.
NOW, if you want to add high-end functionality, that's a few extra bucks. PERFECT. I am all over that, and I WILL be purchasing some extra functionality for this plugin, and with pleasure.
They have structured it in such a way that it's not "crippled" until you pay up, it is whole and complete without any additional stuff. This means they put a lot of effort into making the base (free) stuff work really well.
I can't tell you how frustrating it is to work with a plugin that SHOULD have certain functionality that's broken until you buy. It seems that they put so much effort into crippling the thing that it doesn't work well even after you pay up.
Kudos to these guys doing it the right way. The goodwill that gets created is more than enough to ensure their success. Good job.