[parasubvert]

Because without a legal contract to cover support, you don't have someone that contractually has to answer your call.

You might not care about this, and many don't. You're running the risk that the maintainers won't pack up shop and stop maintaining the project, and that you & your team will need to inherit the project.

Other way of looking at it, is that if you're a business, you always should pay for your software, even if it's open source. Because you will one way or another: through your team, or through the risk that free riding brings.