My carseat manual recommended the rear middle seat as the safest place to put the carseat, and then the rear passenger side as the 2nd safest.
We have Aidyn on the rear passenger side... I just dont think it would have been very good for our backs to have to reach over to the middle all of the time!
Plus I can understand that if the rear middle is slightly raised, then it may make the seat a bit susceptible to becoming unsteady.