This study found that *on average*, a majority of PHEV drivers in _Europe_ don't both plugging them in, making them no better than a "conventional non-plug-in" hybrid.
But as an individual PHEV owner, you can make it far better than this study says - simply by plugging in whenever possible.
I got a PHEV (the BMW i3, what BMW officially called "an electric vehicle with range extender) as my "entry into electric vehicles" - and in four years of ownership, I used maybe ten gallons of gasoline. And I'd say half of that was "burning it up just so it doesn't go stale". It prepared me to fully commit to battery-electric-only with my next vehicle.