There are a number of reasons to consider Group Commons:
The set of modules available under Group Commons is hard to find on the net, and in each case we have worked on the modules repeatedly polishing it.
Since multiple organisations use the software, enhancements are made available to all organisations using the platform. You simply can not purchase the experience we have codified in our platform.
Group Commmons provides integration unparalleled on the net. We built our software from the beginning as an integrated set of software. If you attempt to put together open source packages to emulate the functionality provided by Group Commons, you end up with a nightmarish systems development project. We know, because we tried that way first.