Think it would be good live if you don't want to have to jump to song mode just to set the speed right for the next track/songpart. Sometimes that's all I use the song mode for. And to be able to lock it and change continously would open up a new world of evil time signatures.
Just like in the old tracker days.. ah yeah.
Not sure which speed setting it should prioritize in case you have doubles in the song info though.
..And as I've said before, ctrl machine for the effect settings like delay speed/feedback etc. Same thing there, the more control the better.