I was referred here from another thread. It's a darn good question, so here are some of my initial thoughts:
I take as a working assumption that while there are various reasons to transition the transportation system both to new tech and new practices, co2 concerns are (in my assumption) an immediate life-and-death global concern, and so logically they impact the conversation more than other concerns (IMO). Thus, in my view, the only way that PHEVs can be a long-lasting use case, and not just a transitional tech, is if they go to zero or negative carbon. The only way to do this is if the fuel is ultimately zero- (or would negative- be the better way to put it?) -carbon though there can I suppose be transitional periods where potentially zero-carbon chemicals used as fuels (eg: CH3OH, H2, whatever) are not presently made renewably. As far as I can tell, most chemicals, if we have to, can be made fully as zero- or negative-carbon-dioxide footprint fuels, but the cost is prohibitively high at present. Whether that cost is inherently high and cannot be changed, or just presently high and subject to change, is something that should be discussed. (The question of inherently versus presently expensive is a point I used to raise in the 90s when folks would seem to try to throw a wrench into battery discussions.)
Aside from choosing a PHEV's non-battery energy storage method, there is the question of the fuel conversion method. Generally PHEVs are conceived as using combustion engines, but I think that a fuel cell vehicle with a pluggable battery can be viewed as a PHEV. Two advantages of fuel cell PHEVs over combustion PHEVs are that they are generally potentially more energy efficient and they do not have the other emissions baggage that might be associated with combustion PHEVs (NOx, etc.). I"m not sure that precautionary principle considerations are being fully properly brought into play to look at FCEV downsides while we make some move toward them, but they do seem at the least well worth considering and understanding. As to greater efficiency, I have joked with colleagues over the last decade or two that I'm waiting to hear more discussion of how avoidance of carnot cycle considerations is a way of describing how or why FCEVs are more efficient than PHEVs.
If PHEVs of whatever sort end up being just a transitional tech, then I wonder if we shouldn't try, somehow, to calculate all the extra carbon footprint that will come about from temporarily deploying 2 or 3 waves of hundreds of millions of vehicles throughout the world that then have to be recycled.
Another thought is sometimes I think back to the transition we went through in light bulbs - incandescent and other tech to CFL to LED. CFL will end up transitional, not long-lasting (as far as I'm aware) and it sure has some baggage which make this clear which make me somewhat regret having used them (mercury, for my main concern).
PHEVs have some big advantages such as using established well-known IP and technology know-how (combustion engines, and perhaps using some of the older complex mechanical transmission tech as well) and allowing for a different path in transitioning companies, workforces and supply-chain. However, due to my own ideas of what the priorities are, I think combustion PHEVs qualify as potentially just a transitional tech unless they address the main concern of co2 emissions and co2 lifecycle consequences, as well as addressing emissions, lower efficiency use of energy, and perhaps other issues.