yep that's true.
the thing is, i haven't made really decent calculations or experimenting, but my gut tells me you get much more weight eventually by starting your next grow earlier. After all, grams per watt is nice to know, but it's actually quite meaningless without the temporal aspect - it should be measured by grams per watts per month. so if lets say we get 10 oz after 3 months, and only 8 oz after two months (dunno if it's valid but can't be too far) - after six months this becomes inefficient - 20 oz from doing two batches 3 months each or 24 oz from doing three in 2 months each.
And even for non-commercial grows, considering other aspects - such as the long time it takes to grow and often having to buy expensive bud before your ladies are ready (can be avoided, but often happens esp to new growers etc.), or the ability to finish a grow to deal with various constraints (summer temps, having to move, stealth and smell issues), and of course the costs of energy and substances, well you get the point - it's so friggin long, why make it even longer.