One virtual hostname
On the web it is essential to have even distributed websites respond under a single hostname. A whole industry of balancing technology has arisen, and the usefulness is obvious: One resource is available quickly from many locations.
For technologies where all servers are equivalent, this can also come in handy, so you can set up an IRC network in a similar fashion. When you are on IRC, your identity is defined on all servers of that network, so it doesn't matter which one you are currently using.
ICQ followed up in the server structure design tradition of IRC, but it actually had problems keeping all servers in sync in the early years (at times there were 15 minutes delay until you see your online buddies from other servers).
Other centralistic systems took similar approaches, mostly because the way how people are identified wasn't supposed to carry a server name with them - also because running all necessary computers to host all users in a single location is a solvable problem.
Even MSN, who decided to avoid nickspace collisions by allowing for appended domain names chose not to put server hostnames there, rather use email addresses for identification. Maybe these technologies actually employ a hash of the username to select a server, which would then always be the same one, I don't know - you tell me.((s))
In the case of Jabber this scenario looks different. Everyone has a server hostname in her address anyway, so why bother trying to let a lot of people have the same one? As long as it is the easier solution, fine, but when it gets difficult to scale, it should be easier to have multiple installations.
In the case of players like Google Talk, they already use the one-hostname-fits-all approach for e-mail, so they expect the same to work for messaging. Given their expertise in designing large decentralized systems, if they can't do it, who can. So they make an extra effort at keeping all users visually under the same hostname.
With PSYC you can do the same, but think about it twice - is setting up a large user community under the same hostname really so favorable?
As with any protocol and technology, the requirement of having a single hostname creates extra complications which aren't really necessary: What's so wrong about having many hostnames? Users even get to have their favorite nicknames, since there are several nickspaces to choose from.
Also, XMPP and PSYC are the messaging technologies which use hostnames in their uniforms or jids anyway - there is no requirement and no gain to share a hostname with anybody else. In fact, your own hostname is something you can express creativity, pride or vanity with.