This is a major point. An AC that uses up only 4% of the engine power on a normal vehicle may be more like 6-7% of total engine power on a scaled-down hybrid engine. The same amount of fuel consumption (say 0.2 gallons an hour to cool the car) would be a higher percentage on the more efficient hybrid -- the relevant figure is the gallons per hour of air conditioning (not miles -- more fuel will be spent running AC by driving slower), but few people will ever look at that.

Using the elecrtic however does offer the advantage of strategic load balancing -- running off the engine more on period of lower load, and running off the elecrtic motor when more engine power is needed. There are many automatic algorthims that could be implemented, even on the older belt-only AC systems, that would spread out the AC load based on engine load, but they have largely not been implemented.