Just a couple of points from a PPL with 20hrs in a full motion sim.
Learning procedural stuff on MS FS may work very well, but the reality for me was that a 'real' sim was rather more challenging than flying a light aeroplane.
This was because one had to learn the layout of a complex flight deck, even to use the basic functions.
In fact, I found the autopilot quite complex, because of the need to think through the parameters in use and in particular what was controlling what
The sim was not a B757/767 nor an Airbus, so the more modern FMCs may be easier to use (or nor as the case may be), but I came away after 20 hours feeling that the sim was a demanding environment, whereas I can take off and land Concorde on my mates MS FS setup, whereas I have it on pretty good authority that this would noit be the case in the Concorcde sim!
Thus, if the terrorists had my level of skill or less, I don't think that MS FS is going help them too greatly. I respect the views of the pros, but what seems 'easy' to them is not necessarily the same for those of us with less training and experience on type.