plus a 3 numbered version is very easy to assign to a lib (you know, like a new param for a function bumps up the middle number, a fix - any fix - bumps up the rightmost one etc... )
That is what $Revision$ CVS tag does, version number is totally different thing IMO.
Sorry, but I disagree on this one. You might either think that an extension "version" defines either its api as accessible by the php kernel or as accessible by the php coder, but that's exactly what version numbers are used for.
The cvs tag is for people that compile from source and have to report an error against a a specific version.