Why doesn't the defense get radio's?

[rypper11]

Supposedly the argument is that more than one person on D shouldn't have the receiver on the field at any one time. In the modern NFL, rarely is one D player on the field for every snap and even if so (like perhaps Ray Lewis) one backup player couldn't cover the same dynamic. But is there really any competitive advantage to a DC being able to communicate via a one way communicator with his entire squad? So what if Shawn Springs hears the signal from GW along with Fletcher? If the communications are shut off once the offense gets set (as determined by an official in the box) does it matter if the whole team knows what the call is?