One issue is that of ad-hoc networks. If users can see and control the wireless card they can also setup ad-hoc networks, for both internal and external computers. So, if they are wired to your network and then have an ad-hoc connection to an external computer ... well ...

Todd

P.S. - Please don't assume that because your field techs are not the most knowledgable that they don't know some of this. Home wireless networks are common and so many folks know something about APs and ad-hoc networks. I know elementary school kids who set up ad-hocs all the time.