oh, i can hear the hornets nest buzzing over the horizon!
Personally, i think that having a mentally disabled person in the CFS is fine, as long as they aren't involoved in operational things such as hose drills, callouts, etc.
By all means, i think it would be fine for them to help with things like weekly and monthly checks and basic helping hand stuff. At the end of the day, for them to be there they are probably CFS nuts anyway, so they would probably jump at the chance to do things like wash the trucks, help with station maintenance and so on.
As long as they aren't too much of a hassle for captains and so on, i think retarded people in a non operational role in the CFS would be fine.