The possible downside, as with any new feature, is that code using the new feature will not run on on previous micro (x.y.z) releases. In this case, one can run the tests using the new feature.

>>> from test import __main__
== CPython 3.2b2 (r32b2:87398, Dec 19 2010, 22:51:00) [MSC v.1500 32 bit (Intel)]
==   Windows-XP-5.1.2600-SP3 little-endian
==   c:\docume~1\terry\locals~1\temp\test_python_3872
[  1/351] test_gramm
...
(or via subprocess

Perhaps no one would do either.

However, the new feature currently does not work on Windows (see #10845 and my pydev post), so it definitely should not be backported until fixed and tested. Which is to say, I currently have to use -m test.regrtest even on 3.2 ;-(.