For Jingle calls it would be useful to be able to ask someone for presence before calling them, particularly when gatewaying to SIP (the use case is that the user types in a SIP address or even a PSTN number to call it, and we need to ask the gateway which Jingle dialect to use to the corresponding SIP user, without having to add them to our roster first).
Relevant mailing list threads etc. include:
http://mail.jabber.org/pipermail/jingle/2008-December/000304.html
http://mail.jabber.org/pipermail/standards/2006-November/013017.html (and some other messages in the same month, which Pipermail hasn't threaded correctly)
http://mail.jabber.org/pipermail/standards/2008-August/019511.html
Somewhat related:
http://mail.jabber.org/pipermail/standards/2008-June/018946.html
http://mail.jabber.org/pipermail/standards/2008-June/018980.html
http://xmpp.org/internet-drafts/draft-saintandre-rfc3921bis-05.html#message-chat