Presence overloading

Seeing the extension of presence in the preceding lines, it might seem advantageous to start adding more information to presence stanzas and extending them further. For example, Marty McFly, who listens to a lot of music, might feel it appropriate to advertise what song he is listening to from his client.

While this is perfectly achievable with XMPP, it certainly isn't a good idea. Presence stanzas form a large proportion of traffic on the XMPP network, and filling in the stanzas with additional information will greatly increase the amount of information transferred on a regular basis. Additionally, other users with presence subscriptions to Marty may not be interested in this information, and therefore it would essential be useless data being transmitted.

If this is something you are interested in doing later on, then for this we have Personal Eventing Protocol or PEP (see XEP-0163).