I agree that making content available on free systems is a Good Thing, but before we go down the path banning all encumbered formats, I'd like to ask if there is an accessible middle ground.
Many unfree file formats are poorly supported on Linux and associated free software systems; however, it is also true that many free file formats are poorly supported on Windows and similar unfree systems. For example, one may have to install additional readers/converters/etc. to use OGG and related formats on Windows that aren't available by default. That is not an insurmountable barrier, but it is a barrier, especially to those people with limited technical skills. In addition, there are many situations, such as libraries and public school computers, where the user is not allowed to install any software at all.
I also suspect that there are many people of the YouTube Generation, for example, that would be happy to make their content free, but only really know how to work with MPEGs.
Rather than have Wikimedia say "X format is unfree, therefore X is forbidden", I would much rather see: "X format is unfree, therefore Mediawiki will automatically translate it into these free formats, and provide all users with several options for which format they can most easily use."
Obviously Mediawiki doesn't yet have facilities for format conversion, but in general I think that is a superior goal to simply saying that unfree formats are forbidden. The legitimacy and technical feasibility of performing such conversions may depend on the details of why a specific format is unfree in the first place, and that is something that will probably have to be looked at on a format by format basis. However, whenever possible, I would prefer to increase accessibility to Wikipedia by supporting (and converting) the formats actually in use, rather than erecting barriers to participation.
If the same content could be made available in both a widely-used proprietary format and poorly used open source format, would anyone object, in principle, to using Wikimedia facilities to convert between the two and then distributing both? I'd much rather we work to improve accessibility by making formats interoperable (whenever possible) rather than simply banning all formats that aren't fully free.
-Robert A. Rohde