Ironically, deep space has essentially 0 ambient light. Only the FS2 nebula, atmosphere missions, and heavy firefight set pieces could physically justify an AO bake. At least those are the same scenarios where the diffuse would be bright enough to see the AO at all (unless you don't use dark as hell lighting settings, in which case realism isn't a concern anyways)
I still think it looks better, especially since shadow mapping is still by and large really expensive.
(At risk of being a hypocrite, I think we should probably stop talking about AO theory... this is mjn's Alastor thread after all)