When clients and other studios start sending you the latest Illustrator EPS files, or start sending through Quark and InDesign templates they may have had in the past, I suspect that open-source principles may have to take a backseat to pragmatism. Not many useful fonts are open-source either.
For instance, the GIMP doesn't allow you to work with spot colours, so working with duo and tri-tones, let alone six-colour work, is impossible.
Also, I'm not entirely trustworthy of roundabout methods of correctly making PDFs for press that involve other products than Adobe ones. OS X's built-in PDF support isn't flexible enough for two-colour work or layered PDFs that may contain cutters and the like.
A free lunch is often not enough to make a meal of. 😉