Is it really cost effective to put a motion system underneath a Full Flight Simulator?
While a simulator with motion simulation is more expensive to own/operate than a simulator without motion simulation it is still cheap compared to a real airliner 

So if the regs say a particular type of training can be done on either a real airliner or a full motion simulator then it is likely to be cost-effective to use the full-motion simulator.

And as a bonus you can run scenarios with a high level of realism that would be too dangerous to run on a real airliner. Emergency training in real aircraft has to strike a balance between training pilots to deal with adverse conditions and avoiding accidents during training.


  Why do we have to have motion systems on simulators? We get motion information from our central and peripheral vision, right?


A pilot gets motion information from 3 different sources.


His body
His vision
His instruments.


Unfortunately item 2 on that list is sometimes unavailable and item 1 has terrible medium to long term accuracy.

In normal life our brain uses a combination of our bodies motion sensitivity and our perhiperal vision to build up a picture of where we are. In the absense of visual information it will continue to try buliding up a picture of where we are but the accuracy of that picture will quickly degrade.

It's one thing for a pilot to be able to make correct decisions when his only source of motion information is his instruments. It's quite another to make correct decisions when his body and his instruments are apparently in conflict and he has no visual references.