The darksider is usually the one with less HP. By promoting SD you force him to tele on top and then u have a first shot at him. That enables you to (i) kill him, if his HP is low enough, which usually is; or (ii) at least push him. If you don't make this shot work, then you are probably not skilled enough and deserve to lose.
And talking from personal experience, promoting SD is a strategy I take quite often, and it works nearly everytime. The only thing bugging me is that I need to skip 10 turns. I suggest everyone posting on this thread (who barely even plays TUS) give this small change a chance and see how it goes, and stop proposing dramatic changes which we all know won't happen.