AFAIA, the only case where PHEV owners routinely failed to plug in was the PiP. In the case of company-car Volts, the issue there was one of economics; the drivers were reimbursed for gas they bought, but not for electricity they used for charging at home, so why would they charge? This should be fixable.
The reason I suggest 20 miles or so as a reasonable minimum, aside from the fact that it can cover the routine daily driving needs of 50% of American drivers (assuming it's all done in between charges which won't always be the case, so the actual % may be greater), is that the extra cost of the car is high enough that most people will consider it worthwhile to get their money's worth out of the car by charging it, even if they aren't ideologically motivated to do so.
It does suggest to me that subsidies and perks should be looked at and maybe modified. Right now, subsidies encourage big battery packs, but that may be the wrong approach; maybe they should give the largest subsidy per kWh to the smallest packs that meet the 20 mile (or whatever minimum AER is chosen for PHEVs) limit, with the subsidy decreasing as the battery gets bigger. That way, the people buying bigger batteries are doing so with the full intention of plugging them in, because they know they'll use it. And it pretty much guarantees that people will make full use of their battery range on a daily basis, thus spreading the limited battery storage across the greatest number of cars. Also providing a smaller subsidy to HEVs that meet some mpg requirement (say 45 mpg) would encourage people to really consider their needs, although with the highest gas prices in the country (and another 5.6 cents/gal. tax hike on July 1st, to add to the 12 cents that went into effect 11/2017), maybe no subsidy for HEVs in California will be needed. Prius sales took off starting in 2004 thanks to gas prices.
We really can't allow HEVs back into existing HOV lanes, as they're already too crowded in many parts of California, but what we could do is dedicate another regular lane to them (rather than building new ones, as we know that doesn't decrease VMT or traffic). Naturally, this would be immensely unpopular, and probably politically impossible at least for now.
Guy [I have lots of experience designing/selling off-grid AE systems, some using EVs but don't own one. Local trips are by foot, bike and/or rapid transit].
The 'best' is the enemy of 'good enough'. Copper shot, not Silver bullets.