If only it was that easy. To implement that flag, the extra instance of XMMS has to be loaded into memory, start up (not bringing up a GUI, but still allocating a bunch of memory), detect the previous instance of XMMS and tell it any remote command passed on the commandline and then shut down.
When 20+ instances of xmms all get spawned simultaneously, this happens:
1) every instance of xmms tries to grab its (big) chunk of memory
2) the Z runs out of memory
3) every instance of xmms halts, waiting for more memory
4) NO instance of xmms ever gets far enough along to shut down and release what it's already grabbed
5) the Z gets stuck.
The only ways to avoid this are by disabling autorepeat on the keys bound to the XMMS controls, or to use a much more lightweight app to capture the keys and communicate with XMMS via it's control API's.