Using the "great" argument of not having to setup or create an account, sounds very nice in a first analysis, as it might seen that you are not getting another service to be attached to.
Well, it turns out that this is not true, first because you are probably exposing the information and account settings from the previous service to the new server, the only difference is that you don't need to do that manually.
But what really makes it really worst, is the attachment factor, that is created on interdependent services like applications inside facebook, orkut, twitter etc...
Once you are using a service that depends and requires other, and this service is really important for you, it attaches you extending you level of dependency through the overhead of being stuck sometimes with services that you no longer need, or never needed.
Think before getting attached to interdependent services. Make also sure the level of account exposure is really defined in a proper and clear way, as most of these integrations are not clear at all about account exposure among different services.
XMPP can support your freedom by exposing all these services through a protocol, which does the integration in a open and defined method, making you able to choose and define dependencies and exposure of your data.
In that sense you might even start your services through Google Wave, and later decide to stop and provide through your own XMPP network. Or even having Intranet services integrated in a safe and limited network.
Powering your freedom!