I based my statement on yield factor and pitching rates. If you pitch 1 vial In 1L you'll get a yield factor of 7.4. If you pitch 1 vial in 2L the yield factor is 7.6. Remember the higher the yield factory the better the growth. And the yield factor from 150 million cells in 2L is about 4 so you will not create another 100 billion cells you will create about 40 billion cells because of the inoculation rate. "As the inoculation rate drops the yield factor climbs." Quoted from Yeast the proper guide to beer fermentation.
I understand the math, though my numbers are more in line with what sjbeerman says. 205B cells from a 2L starter and 266B cells from a 1L to 2L starter. I understand that it doesn't double every time you step, but that is a significant enough of a difference that it is worth noting. Even if it were just 40B, as you cite, that's still a 20% difference. It may not be necessary for any particular beer, but I wouldn't say that 200B is a better result than 266B (or even 240B).
The talk of yield factor in the Chris White book always struck me as odd, and I don't think it means what you are suggesting that it does (assuming I'm understanding you right). A yield factor of 7.4 on a 1L starter vs. a 7.6 on a 2L starter means that the 2L starter will net you approximately double the yeast growth of a 1L starter.
Higher yield factor does not mean better growth. In fact, there is some talk that the opposite is true. Yield factor is a ratio of growth to sugar consumed. Why is that interesting to home brewers? I'm not sure that it is...I'm very interested in growth numbers, but don't really care about the amount of DME I need to use. Maybe that's wasteful, but it strikes me as kind of like spending an extra hour on your sparge to save 60 cents on grain. I understand why it would be an important consideration for big time brewers for whom a few points equals millions of dollars, but DME is cheap.
Anyway, to each their own. I certainly don't want to sound like I'm suggesting that stepped starters are always better, but nor do I think it is fair to tell someone doing a stepped starter that he shouldn't because a single will give him better results (in the absence of any of the important details).