First, note that command= does not invoke a SSH subsystem. It merely runs the command as if it were given on the SSH client's command line; e.g.
ssh yourhost "svnserve -t --tunnel-user=alice" ssh yourhost "git upload-pack /pub/git/myproject.git" ssh yourhost "ls -la" The above examples should make it clearer that communication with svnserve or git or ls happens over the same stdio (stdin/stdout/stderr) as any other SSH interaction. For SVN and Git, the ssh client merely serves as a tool to invoke commands remotely.
"Real" subsystems, as configured using the Subsystem option in sshd_config, aren't that much different. The only major difference is that they can be invoked by a static, well-known name, instead of relying on the remote login shell (bash, zsh, &c) to find the correct executable. For example, the SFTP server can be /usr/lib/ssh/sftp-server in one machine, MULTINET_COMMON_ROOT:[MULTINET]SFTP-SERVER2.EXE in another, built into sshd in third (Subsystem sftp internal-sftp), but in all cases clients can still find it using the name sftp.
At least in OpenSSH, subsystems can be written just like normal programs that communicate with the client via stdin/stdout/stderr. It seems that passing arbitrary command-line arguments is not allowed, however, so you cannot just configure a single svnserve subsystem for all users.