> What uses cases involve running new untrusted code on legacy unsupported systems?

Well yes, you shouldn't be doing that (particularly the 'unsupported' bit). The thinking here, I suppose, is that Meltdown makes privilege escalation straightforward once a malicious party has access to the system via another vector.

In my company, we run self-hosted physical servers (i.e. we're the only user on the systems) and debated whether to disable the page table isolation fix since we were seeing about a 30% performance hit.

The decision we took was that we would accept the performance hit since Meltdown means that any unauthorized entry into the system has a privilege escalation path since kernel memory is essentially readable by any process. (Quite an easy and quick decision, really.)