Interesting question.
A bridge plate has been used by some classical guitar makers (Hauser) and not used by others (Hernandez). Romanillos said he felt it created stresses in the top and others have felt it "balances" the construction (similar to plywood).
I - generally - use them when I use a pinless bridge and for a steel string instrument I think it is prudent. If the pinless bridge is a failure replacing it with a pinned bridge would not be too difficult.
Another advantage to using it is it can provide some additional structure if mechanical fasteners are used to counter the peeling force on the bridge. On some of my steel string pinless bridges I have used tiny bolts bolted through the top and hidden under MOP dots ( the nuts are epoxied on because the vibrations of the top will make them come loose
). On others I have increased the width of the bridge to help counter act those forces. Classical guitar bridges seem to survive O.K. without any additional "anchoring" measures, some in fact have done O.K. with narrow "lute like" bridges.