Anyway, you won't be able to migrate your account to another provider at all when using most of other networks, and generally you shouldn't need to do that with XMPP as well (only in rare, special situations), so I really don't think that every user-friendly XMPP client out there needs to provide big fat "MIGRATE MY ROSTER" button.
This pattern of thought is why it still isn't the year of Linux on the desktop. Black squares containing only text scare people.
We seem to have quite different views about "common people".
In fact, to just one-time migrate your roster you don't even need any of them - you just download the roster from one account and push it to another. There are plenty of tools to do that on the Web. XEPs make it easy to keep them synchronized, or to "push" roster from one account to another without having to log in into new account with the tool you want to use.