Should the first list include –quit too, as that one also doesn’t seem to work? It (2.96) doesn’t print a warning though, either:

jani@saegusa:~$ rhythmbox-client –quit
jani@saegusa:~$
jani@saegusa:~$ rhythmbox-client –debug –quit
(18:28:03) [0xc97800] [rb_debug_init_match] rb-debug.c:240: Debugging enabled
(18:28:03) [0xc97800] [main] rb-client.c:707: quitting existing instance
jani@saegusa:~$