Quite simply, superdelegates are a dangerous thing. It creates a potential for the power to be taken away from the will of the people.
Basically, individuals tossing their weight around is no basis for a system of government.
Supreme executive power should derive from a mandate from the masses, not from the whims of a select few individuals.
Essentially, the superdelegates are people. People who have the power to influence elections to make sure that who their party wants in, gets in.
Just having them exist, is a blow to the democratic process, because it makes people feel like their vote doesn't matter, because their vote can be ignored, at the whim of an individual they have no control over.
Superdelegates can rig elections. It's a broken system. Fix it.