On Stack Overflow and Meta.SE, each of which have their own chat server, this makes sense. However, the other 150 sites on the network share a chat server, where it's more complicated.
On the shared chat server a chat user has a parent account, an account on one of the sites. Sometimes that account gets deleted, leaving an orphan chat account. That's messy. But we can't assume that the user has no other accounts. A deleted (as opposed to abandoned) main-site account might mean the user was deleted network-wide (in which case we also want to delete the chat account), but it might have been more localized. What we don't want to do is to nuke a chat account that somebody is using on Programmers just because he deleted his account on Philosophy and it happened to be his chat parent. (Chat parents are assigned automatically and most people probably never touch or even notice them.)
The root problem is that when a chat user's parent account gets deleted, the chat account doesn't get reparented. We should fix that, and delete the chat account only when there is no remaining account on that user's network profile that is relevant to the chat server in question.
How we choose a new parent doesn't matter all that much, as the user (if still around) can change it if he wants. Choose any of: oldest account, then-highest-rep account, random.
(Possibly related: if we don't already, we probably ought to delete the network account when the last site's account goes, too.)