Since we are having 1/12 and 1/16, maybe it's a good time to add more playback rates for mappers. It's really hard to check if 1/12 or 1/16 is needed for the song with 25% speed.
I mean the thread has a different reasoning than yours, but either way I don't think it's necessary. Logically speaking, it doesn't matter whether we have 1/12 or 1/16. All this means that we can use lower tempos to map faster songs. It doesn't mean we will have faster and faster songs. Thus, it isn't really necessary since people have been timing things fine without it. Also, there was some discussion about the unusefulness of lower than 25% playback rate in that thread above.