An extra midrange driver would absolutely be more beneficial for increased dynamic range, however supertweeters can help with dynamic range. They can allow you to use a larger or heavier tweeter when it doesn't have to worry about high-treble.
Look at this big bastard, for example. That is a 8" AMT tweeter. It will have massive dynamic range from 3kHz to 10kHz, but its response falls off above 10kHz. That isn't a problem if you can cross over to a supertweeter.