Free has more appeal to developing a custom version in your spare time to suit your needs.
A paid script - normally means better support but also means you want modifications done - which generally cost per mod if you can't do it yourself.
A free script means you can try out all the other free mods that communities develop for them - eventually you may find yourself only using part of the script - stripping it down and having a nice custom
website hosting for free. Obviously not totally free as you would have spent a lot of time developing it - but it saves getting burned by some of the useless paid-for scripts out there.
One that wasn't mentioned and I have seen an increased community development was joomla - I never used it personally - but they do have a social mod from what I remember reading.