Every time you introduce a new driver, you have phase and response issues. For transients, "time alignment" etc is important. Keeping the transients intact is difficult if you introduce a crossover and another driver right smack in the middle the audio spectrum where it really affects things (5khz on up).

Now if the crossover is down lower, where the wavelengths are longer, it is not as difficult to keep everything in line. AND if you can make it a 6db crossover, you are doing even better. Now, the issues here are power isolation for the tweeter, and if the crossover is low, a driver that can go low will usually not be able to produce the top octave.

Well, Motorola piezos to the rescue... no problem with low frequency energy as the impedance is very high down low, and the tweeter does not produce these frequencies. Also, these tweeters have plenty of top end range... in fact, in excess of what is really needed.

And on top of everything else, I like the KISS approach.