Why Ubuntu's PAM versioning failure matters
This PAM ABI incompatibility was not introduced by the upstream package (in a patch or a minor version update); it was introduced by Ubuntu's own patching. Ubuntu makes a big deal out of doing only minimal changes in their LTS releases (so minimal that they generally will not fix bugs, only security issues, as far as I can tell). This means that Ubuntu should be carefully auditing the patches that they use.
(If Ubuntu is not carefully auditing patches but is still beating the 'minimal changes only' drum, they are either being lazy or are extremely short of resources. If it is the latter case, they should admit that Ubuntu and especially Ubuntu LTS receives minimal care and attention because they don't have the manpower to do any better.)
That an ABI change slipped through anyways reveals a process failure of Ubuntu's patch auditing. This failure is (probably) the real root cause issue in the PAM versioning failure, not the cron locked up due to a PAM update or even that PAM had an ABI change and it was not detected.
If Ubuntu has had a process failure in PAM patch auditing, it can have a process failure in patch auditing for other packages (and indeed it seems to have had in some cases). Failures in patch auditing are serious; just how serious is revealed by the Debian SSL debacle. If Ubuntu's patch auditing is flawed (or simply only superficial, or even absent entirely), applying Ubuntu package updates in general (not just PAM updates) may be significantly more dangerous than people thought.
Sadly, it's not encouraging that the only actions Ubuntu people talked about in the Ubuntu bug were surface level issues. And really, that's the final reason that this versioning failure matters; it may have exposed something important and disturbing about how Ubuntu people approach maintaining Ubuntu.
(Yes, I'm taking kind of an extreme case view here; the reality is likely to be less scary. But we don't know, and it really does irritate me that Ubuntu's public reaction was so superficial and that people were so accepting of it.)