Personally, I like a wide midrange band to cover the whole vocal range, better if covering the whole piano range. In practice, I tend to choose a driver that covers as much of that as possible, but all my systems are multi-way, so they have to crossover somewhere. Single driver systems don't have the crossover, but that means they have to choose between bass excursion and high frequency extension. The cone has to move enough to make some bass, be rigid enough to limit breakup mode peaks in the midrange, but flexible enough to have some controlled modes for top end extension.