From my own testing PLot Landmarks suppress automatic jams even tho the client shows you snapped to the nearest road. Given that, the only arguments for mapping parking lot roads are:
1) Gives Waze a segment closer to a POI/Address that's more appropriate to navigate to, e.g. a Mall/Office building next to a freeway without the roads may navigate to a point on the freeway. Also allows navigation to the unit for strip-malls, which is pretty cool actually.
2) Can give the best route OUT of a large parking lot assuming turn restrictions are set correctly.
3) There's turn restrictions/signage at exits that disallow certain maneuvers, such as exiting a parking lot via right turn, cutting across 3-4 lanes of traffic to make a left at the traffic light 15ft away from the exit. This currently still needs hacks to prevent, see: here
In either case it's usually the larger lots that need actual segments, the corner gas station or mid-block drive-thru usually just needs a landmark.