AC upgrade on 73 Sport

Vehicle is a 73 Dart 340 Sport, factory AC car. I'm upgrading the AC as part of my rebuild. Already have a Sanden compressor that came with my CVF pulleys. The heater box has been rebuilt with new heater core and evaporator. I was looking at the Classic Air upgrade and told them I needed all the underhood parts except the compressor, and that the interior was rebuilt with a new stock evaporator. The guy said I had to replace my new evaporator with one of theirs. Not being an AC guy, can anyone tell me why I have to replace a brand new evaporator as part of the upgrade?