tax incentives should apply

Yes, but as an ICE car, not as an electric, I think most countries have this by now, that cars are taxed lower if their mileage is better. But for electric it's even lower, because they drive on partially renewable energy from the power grid, and they don't pollute in cities. Those factors don't apply for Hybrid cars.

Many people with plugin hybrids only add gas a few times per year.

Yes and that's the basis for their tax rebates here, but the research shows that the level of gas used for hybrids is way higher than manufacturers promised. People generally buy hybrid because they have recurring situations where they need the extended range the gas provides. So in general a hybrid is driven quite a lot on gas. If you don't have that need, you might as well buy an electric. Ergo Hybrids should at most receive half the tax rebate IMO. And they should never be counted as electric, because they are not.