Digium is unusual in that in addition to distributing Asterisk under the open source GPLv2 license, it also makes the software available with a low-cost commercial license. This provides one final reason to pay for open source software: If you pay for a commercial license, you can modify the software without the obligation of providing the resulting code to the original development community under the GPLv2 license. This can be useful if you want to incorporate the modified code into your own commercial products.
That's a (Commercial) Wrap
What you are generally paying for with a subscription to an open source-based product is a commercial wrap to put around the open source code. That wrap includes support, testing, hardware certification and predictable product lifecycles.
"By paying a subscription you get the same experience as you do with proprietary software, only for far less money," concludes Haff.
Read more about open source in CIO's Open Source Drilldown.