It's kind of pointless. If the police have already identified you as a target, there are too many other tactics they can use. For example, they could just set up a hidden camera that records you when you are using your PC.
Well for one this is a post in response to the Silk Road arrests in which a DMS would've saved him. Not pointless at all. Sure over time their methods will evolve but ask Ross if he'd rather have had a DMS or not. It'd be unequivocally yes. If your point is 'if you're a big enough target, you're pretty much fucked whatever you do given enough time' then yes that's probably true, too.
If they observe the computer locking upon you removing the FD, wouldn't they induce the connection? Unless it irrevocably makes the system inoperable, I don't see this as solving for the larger problem.