It also of course needs the resistive heater when driving EV. However in HV mode my engine doesn't take anywhere near fifty miles to warm up, more like about five miles even during winter. Once the engine warms up and the thermostat opens it starts sending warm coolant to the radiator to dump the heat overboard into the atmosphere. Meanwhile at the same time that it is dumping heat overboard it continues to exclusively use electricity to warm up the cabin. Even though they have built in the capability to send engine coolant into the cabin loop, it apparently doesn't do this unless the resistive heater can't keep up on its own. One article that I read said it only uses engine coolant below about 5° F, although I don't know how accurate that is.
What it should do is the opposite, use engine heat when available, and use the resistive heater only when engine heat isn't enough. I'm sure it takes a little bit of effort to modulate the heat coming from the engine, but ICE cars with climate control do this. Maybe it's a little more complicated in HV since engine temperature will be fluctuating a bit as ICE shuts of momentarily at times, but with modern computer controls that doesn't seem all that insurmountable. That's how my Prius worked.
I suspect the reason they didn't take the effort to do this is because it probably doesn't affect the EPA rating so it wasn't a priority, the priority was maintaining steady cabin temperature with the least amount of effort. Modulating electricity flow to the resistive heater is probably easier than modulating coolant flow with a valve, because the valve would have to be built robust enough to function for many years. To be fair they spent a ton of money on this car in other areas so I can understand that at some point they have to go with what is simplest.
Along similar lines, the climate system blends in outdoor air to maintain temperature. Of course there is always some fresh air coming in, but the system brings in additional outdoor air when needed to keep the temperature in line. For example if you are running the resistive heater to warm the cabin, and the temperature swings a couple of degrees too high, instead of simply cutting off heat temporarily until the temperature naturally lowers, instead they mix in some cold outdoor air into the cabin as this will much more quickly bring the temperature in line. But of course at a cost since bringing in cold outdoor air will push warm cabin air out of the car through the vents, cabin air that you spent your hard earned dollars heating up with a resistive heater.
Another annoyance is that you can't set the system for cold or heat, it does whichever is needed. While convenient, that creates situations where you have the temperature set to say 72 in the afternoon, which runs AC, but then in the morning if the cabin is say 68 degrees, then the heater comes on to bring it up to 72, when you don't want the heater to come on. I realize it's just doing its job, but in regular cars if you have it set to AC that's all it does, and if set to heat then that's all it does. If you want heat you switch to heat.
Some of this may not make a huge difference, but cumulatively it can. All cars are less efficient in winter for a variety of reasons, but the Clarity really seems to get slammed during winter and I suspect this is part of the reason. Climate should have its own "Eco" setting for those who don't mind a little bit of temperature fluctuation as a tradeoff for efficiency. In the owners manual Econ driving mode is supposed to affect climate control, but my guess is that it just widens the temperature range a tiny bit that's all, it doesn't do any of the things that I am suggesting.