A properly wired generator that feeds a residential service uses the service ground. However, supplemental grounding is a real good idea, especially if it's connected all the time. Connecting a ground wire during a power outage isn't usually what's on my mind.
The issue here is that if a generator looses it's ground, its common potential floats and can be different than ground potential. That means that somebody inside can get zapped when touching an appliance and plumbing for example. That's also why backup panels must have 3-point breakers in my area. Repair linemen get knocked off poles by improperly wired or grounded generators that can back-feed through the neutral line in 2-point breakers.
In my hookup, my feed runs under-ground from a shed behind the garage. I put in supplemental ground plates for the garage and all outbuildings. The generator ground is wired to the ground circuit in the garage wiring. With this connection, the generator gets its ground both from service ground through the feed-line as well as through the generator's ground connection. The generator ground is pretty bomb proof, and also, a ground is already connected if I want to run an extension cord from the generator without connecting the backup feed line. Multiple ground paths are a minor efficiency consideration, but multiple ground paths provide better protection against lightening.
It's still hard for me to believe that a licensed electrician would terminate a generator feed line in a receptacle, rather than a plug, and not even a twist-lock type.