Traffic on a busy city street moves more swiftly, reliably and efficiently if everyone is moving in the same direction. Also, traffic lights, when appropriately placed, speed traffic along. At one time or another, you've all been frustrated trying to get through an intersection when the traffic light was out. So, too, spectrum bands are generally most efficient when the services within the band are similar. More insulation is needed to separate incompatible uses.
Equipment becomes more expensive if it is manufactured for limited bandwidth.
Our critics accuse us of bureaucratic paternalism for suggesting that some guidance as to expected use of the spectrum is welcome. But this isn't a debate between ideologies and overzealous government bureaucrats. It's a debate between those who decide on the basis of theory and those who listen to what the manufacturers and service providers, inventors and investors are telling us. We tried total flexibility in conjunction with the GWAC's band at 4.6 GHz. It is two years later and still no one wants to play.