It's impossible to concentrate when you've got alerts popping up on your phone or laptop every nine seconds. We all know the feeling of being deep in conversation with someone and then seeing them glance at a text message; the energy wanes, and you're thrown off track.
This happens because whatever link your minds have established is now broken. The person you're talking to suddenly prioritized a different conversation and left you hanging.
If it's hard to recover your sense of connection and momentum when it's just you and a friend, a whole tableful of distracted colleagues is orders of magnitude worse. Nip that in the bud, and outlaw any device not integral to the task at hand.