Why put "sync-message"-ed messages into the async queue of bus?

[Victor]

Is it redundant to put the message that has been processed by the 
callback of signal "sync-message" into the async queue of a bus? There 
isn't a way to set the value of variable `replay` (inside 
`gst_bus_post`) to `GST_BUS_DROP` if the message has already been dealt 
with in "sync-message" callback.

What is the purpose of it to let each message be (possibly) processed 
twice, once in "sync-message" callback, once in whatever function that 
takes the message away from the async queue?

Thanks, and sorry about my English.