I've worked on the software of a lot of big energy companies. A lot of those companies can make tons of money just because they are huge and have deals going on everywhere. This gives them a lot of information about what's going on in energy markets all around the world. In short, this information is highly valuable, and yet it often flies around corporate networks in plaintext. Just undetectably getting this information to an outside party would be a highly illegal act that would enable a great many other profitable and highly illegal acts.

(And it would also be very hard to do this without getting caught. Even harder than most people would think. Particularly at the point where you're trying to make money.)

Probably because there's nothing new here. Controlling physical access to a network is a long standing problem with a wide variety of solutions. As with any security technology, these solutions are imperfect, but the plug computer only brings a couple new factors to the table: it doesn't look like a computer and it is extremely small, so it's very concealable.

Neither of these contribute significantly to your exposure any more than something like a laptop or netbook. Concealment helps protect against casual detection, but many offices are littered with spare laptops that no one pays any attention to any way. This is especially true in IT departments where systems may sit on shelves for days or weeks waiting for repair. The notion that a plug computer may be hard to find once identified sounds scary, but in practice, it's a non-issue. You simply identify the port to which the device is connected, unplug it, then trace the cable back to its termination point. I'm going to ignore wireless, because no sensible security plan involves a WiFi network attached to their private network. You segregate wireless in the DMZ, then allow WiFi users to connect to a VPN endpoint using strong encryption.

Any business concerned about securing their networks has implemented policies like shutting interfaces that aren't in use, and authenticating access at the Ethernet level using 802.1X. Neither of these are foolproof, but depending upon how secure you want to be, you build up security at every layer: physical, Ethernet, VPN, application.