There are a significant number of 'missing features' in the free version of Unity3d...for example, render-to-texture. That's a pretty serious omission for any kind of serious software development - so the $1500 (or $75/month with a 2 year commitment) is necessary if you are really serious about game development. In a typical game company, $1,500 is roughly the salary of one programmer for a week. So over the life of any reasonable commercial game, the cost of buying a full license for each worker is essentially negligible.
What the free versions do is to enable indie studios to grow to the point where they can afford to pay for a game engine - and to get amateur game developers to grow interest, loyalty and expertise in a particular free engine that will hopefully translate into sales of the professional version when they become paid game developers in the future. But there are enough annoying road blocks that even an amateur developer may be tempted into buying (or renting!) the full version after running into a few of them.
It's a good model, and I hope it grows and continues.