... since it seems reasonable to expect more substantial changes with more substantial version number increases.

That seems reasonable to you. The same number means different things to different people. Certainly you know that Perl 5.15 is a development version, but tell a thousand Perl 5 programmers who aren't tied into PerlMonks or perl.org or IRC or YAPC or Perl Mongers that Perl 5.15.2 is out and see how many of them say "Cool, how can I get it?"

Perl 5 already had a version number change between 5.005 and 5.6.0. That change meant exactly what the documentation for the version number said it meant. The same would go for any other change.

The notion of perpetual forward compatibility (where use 5.12.0 means "Anything 5.12 or newer obviously must work!") has its obvious flaws. Fix that—that's one of the real problems, after all—and a lot of the handwringing over the meaning of version numbers can go away.