dev-vcs/subversion installs per default svnserve. It you just want to use only the client, it's unnecessary to install those files and scripts. That should be covered by the "server" use flag.
There is also installed a xinetd file, which should be only used by the "xinetd" use flag.
USE=xinetd is invalid. USE=server may be valid but I'm not sure if the effort needed to disable server components is larger than the cost of building them.
Re-assigning to the remaining maintainer.
Patches would be welcome...
Created attachment 443172 [details]
Maybe something like that?
Created attachment 443174 [details, diff]
Unfortunately this patch still does not prevent compilation and installation of svnserve binary. I've looked into the build system and I see no chance to prevent compilation of svnserve.
Closing, since I don't think this will be ever somehow patched..