I think the drawback with allowing them all to have their phones in class is that you have no control over what they're actually doing on those phones. There is no way for you to limit their use to online dictionaries, research sites or anything similar.
There is no doubt that they will use online facilities when they're doing their homework (at home!) but I honestly don't see a reason for them to need them in class. During class they should be reading, writing, listening or speaking.