When waterproofing my basement, should I apply 11 mil plastic in addition to the dimple board?
More stuff doesn't always mean better. Sometimes it means more expensive, or sometimes it means overcompensating for other weaknesses in the structure. Or sometimes it means old contractors who just don't want to give up their stubborn ways. 

In your case, the waterproofing should be the parging layer. That should be some form of rubberized sealer that forms a complete barrier. The dimple board is there to protect that layer from backfill and to provide a drainage plane to the french drain. 

Provided those are all done properly, I can't see the plastic adding any value to the equation. Getting a foundation wrapped watertight with plastic sheets doesn't seem viable either from an installation POV. 

So, my vote is that it's unnecessary.