> I can't think of a really good legit reason for wanting a cross-app unique ID.

The use case that's affected the most by this change is that of advertisers who want to track the performance of ads they buy from publishers, which I think is a legitimate need. The fact that the IDFA uniquely identifies an iOS device (unless the user resets it) is a side effect of Apple's implementation of it; all app advertisers really want is the same kind of first-party conversion tracking for their apps that's standard everywhere else on the web.