That's true to a point, that's why a lot of people burn up alternators because of running too much power at low rpm and saturating the field. We don't typically idle around so the alternators RPMs at cruise are in its efficiency area so there's no point in having all this power unless you are just idling around all day. I run in half in town and half freeway and what the alternator can't supply it takes from the battery like on fan startup. The alternator can easily replenish this.
Jeff