If anything, a PHEV that is never charged is actually worse than a HEV because it carries around a few hundred extra kg worth of batteries and charging equipment. That's going to lower fuel efficiency a bit.
I do know that PHEVs purchased for commercial motorpools tend to see low plug-in rates because the drivers don't care. It isn't as if the drivers are saving money and enforcement can be a cumbersome.
And some people bought PHEVs back when they still qualified for carpool access, even if they had no ability to charge. I have a neighbor with a Fusion PHEV that did exactly this.
There are also a few exotic PHEV sports cars with little electric range that mostly use the electric powertrain for low-end torque improvements. If your car only gets 15 km in EV mode, I can see why you might not always plug it in.
I tend to limit my PHEV's charging to overnight at home and the occasional free charger at work or while shopping. I could charge more often, but daytime electric rates and public charger rates discourage it.
Something else is range. My PHEV gets about 75 km on a full charge, which covers about 95% of my family's typical in-town driving. I looked at PHEVs that only had around 50 km of range on a full charge and couldn't justify the cost vs. payback period since our all-electric coverage would drop so much. You'd also need to be more habitual regarding charging. No thanks. I'd rather just get a HEV.