I buy it because it does something that I need to do. I have yet to see a single piece of software that doesn't require any learning to use. Whether it be 10 seconds, or 10 days, there is always a learning period involved. And often one finds there are more efficient ways to do things within the software than then they may have used at first. In that case, you've just spent time learning to use it more efficiently.
Some software costs money.
OSS tends to cost learning time. I am limited in both, but for me, learning time is better spent. I'm not a developer, I'm a network admin. If I spend money, I get software and time in return. If I spend time, I get software, saved money, AND better understanding of the systems I work with.