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.)
On an unpatched system, that's not likely to be hard in the first place.