Actually some people have done some gas usage tests and found that HV Charge is pretty efficient with little impact on mpg. It of course uses more gas during the time that HV Charge is running, but it is storing up EV range to be used later, so the overall mpg ends up about the same. HV Charge should be used mainly on the highway if possible where it can be more efficient since the gas engine is already running, whereas if you use HV Charge in city driving then there will be times when the gas engine would normally be off, like slow speeds or when you are stopped at a light, but if you have HV Charge turned on then the engine will continue running which is not going to be as efficient.
There are situations where it would be nice to drive in EV but for whatever reason your EV range has gone to 0. HV Charge is a convenient way to build back some EV range, with only minor loss of efficiency. It can be especially helpful on long trips where you don't really have opportunities to charge, at least not conveniently. It's no fun being on a trip and driving all the time with 0 EV range. HV Charge lets you build up some EV miles while cruising on the highway to be used later, making for a more enjoyable drive.