I've thought about that as well, but it has two disadvantages:1. AI cars will spawn using both, half of the time it won't have the right plate. 2. You lose extras you could use for ELS or other parts. If you do it properly, you need one extra for each plate, as they're often connected to moving parts like the the trunk or bumpers that could fall off.
I think with extra part he means for DEV cars. So that you have a templated plate beneath the normal plate, so you just need to delete the normal one in zmod so you can use the templated plate.