>do not give valuable information
I was able to debug an error due to an old version used (the one for the wiki) thanks to the svn version. Perhaps we should make up a new system.
We do have a system. It's why we pass __FILE__ as 'path' to $wgExtensionCredits. Special:Version uses this to find the .svn directory and output the information. Granted, this only works for wikis based on a SVN checkout, but it's the best we've got for now. The other thing that developers can do is have meaningful version numbers, and actually increment them when changes are made :)
If you've got any other ideas on how to keep track of SVN revisions without using keywords or a post-commit hook to modify something, I'm all ears.