At the end of the day, a kensington locks doesn't offer any actual device security, it's just a small deterent, it takes all of a few seconds to break it.
If you really need a deterant, you could look at one of the usb locks. They don't really do much for actual security, but it is a deterant, and the would-be thief would probably realise that by forcefully removing it they would most likely be breaking the usb port instead of just a lock slot that doesn't actually effect the use of the device.
The cable lock certainly isn't 100% security any more than locking your car doors.
The cable can be cut and someone can throw a brick through your car window and get inside your car locked doors or not.
A cable lock will prevent low key theft where someone casually grabs it off a desk or out of a docking station acting as it they own it.
I thought of one possible workaround that might allow us make this work for us in an environment where you are forbidden from leaving any computing device whether or not it is a "tablet" or "laptop" unattended and unsecured.
People will be turned off by the thought of having to put their Surface in a locked drawer every time they step away from it and it will be unweidly to always have your hands full with it if you carry it with you as you walk around the office,
They might be able to work with using a small messenger bag with a shoulder strap that would allow them to keep the Surface with them without having to constantly hold on to it.
Not sure that will be deemed acceptable by the people who will make the decision, but that sounds like the least-worse workaround of what's available.
The ones in the photos may be sized for iPads. Maybe after the Surface 3 is on the market there will be a good selection of carrying cases like these that are the right size. People people do not want to deal with carrying around oversized, bulky bags, but it needs to be just big enough to carry the Surface 3 with Type Cover and the charging brick.