According to the ARIA spec, several ARIA roles should have their aria-live implicitly set, e.g.: http://www.w3.org/TR/wai-aria/roles#log role="alert" -> aria-live="assertive" role="log" -> aria-live="polite" role="status" -> aria-live="polite"
Do you have a test case where this doesn't work? My recollection is that these work as expected without the need for an explicitly aria-live value.
I can't repro. Looks like the code is implemented correctly.