At our level, i.e. below coder, the only critical upshot to open source is its corollaries: its free to use, upgrade, fiddle with, all without paying out the nose. I run a server off of Slackware 8 that cost me $0.27 in burnt CDs: mail, web, its a NAT for a house LAN, ftp, and a bunch of other stuff. Same setup using comercial products with more bugs and no support forums like this: 2, maybe 3 grand; plus I couldn't run it on an old P Pro 200.
Hope that makes sense,