The Navy is doing this for the same reason F6F Hellcats aren't being used as "adversaries", to have modern aircraft in training. Rev A and Rev B F/A-18s were what came into the fleet in the 1980s. Their capabilities are not current, maintenance is probably needed more frequently, and some spare parts are probably getting hard to find (e.g., does AMD still make 2900 series bit-slice processors, which are used in the original MDI?).