Well, adding tests is certainly useful to avoid regressions in CPython (not only PyPy or Jython). It seems to me that the only reasonable discussion is whether they should be committed to 2.7 and 3.2, or only the default branch.

(I personally like adding tests to stable branches as well, since that avoids potential regressions in further bugfix releases)

If we had had such tests from the beginning, 3.x wouldn't have regressed in that matter (see #13848).