the server can't take a lot of that
If bandwidth is a problem there are a lot of good professional and free project hosting around the net. Sourceforge and Google Code being the first that come to mind for subversion.
has little benefit for the player
Most players just get the binaries and are happy, however relying on a version control system is meant to help the project as a whole rather than just end users. It helps development and releasing.
Tag branches would make that situation worse too
I'm not sure how a clean and readable repo would make things worse. It would lessen the bandwidth required for one, since someone just wanting to develop could just grab the trunk rather than downloading every release OA has ever had.