SASless,
What you hint at is the difference between "training" and "checking".
Speaking from a non-regulatory standpoint, a good quality FTD can provide almost all of the systems training you could ever want (and if it has a good base shaker probably most of the flying training as well), while a higher fidelity device is probably desired during "checking".
The cost to benefit ratio of having a motion system is debatable. A hexapod is pretty limited in terms of the cues it can produce, so there's always a certain amount of misleading cues that get generated during the more aggressive maneuvers. A good argument can be made for bagging the motion system entirely and just going with a good vibration platform.
But all that is moot because the regulatory agencies dictate what level of device is required in order to receive "official" credit.
If a hard case could be made to the regulators that time spent in an FTD (which can cost nearly an order of magnitude less than a FFS) is just as effective as time spent in a FFS, then they might be willing to listen. But the process of creating a whole new set of regulations has just finished, so I don't think people have the energy for revisiting things at this point.