I have been wondering the same thing. It occurred to me that if we used the
<a4j:poll>
to only be enabled if the messages were there
#{not empty facesMessages.currentGlobalMessages}
And then the poll could wipe out the messages. Kind of a one-shot process. More likely we should have a button to "hide" the messages or clear them.
I think users might be a bit upset if they turn around and when they come back they are gone :-)