It works when your employer uses the software you create. It's a lot tougher when you want to create something that no single employer is going to get enough value out of to justify the total cost.
That's where you want something that allows you to pool customer's money together.
Dries Buytaert sees a need also:
Open source is the best way to build and distribute software. There are few things about which I'm more convinced. In some, but not all cases, Open Source also offers a viable business model. When it does, it's great because it allows you to do well and do good at the same time.