Efficiency loss from skipping a sparge depends on how strong the beer is. Light beers use a small amount of grain, so loss of wort to grain absorption is smaller than for a strong beer.
I haven't found your first point to be true. In a high gravity brew, you will lose a lot of efficiency even with a sparge but with no sparge or a "poor" sparge where I only pour a small amount of water over the grains for 1 minute my efficiency has never dropped below about 55% even for a big beer.
Also I too regularly do no sparge for small beers simply because my efficiency is already close to 90% with small beers so there's no point in making it any "worse" by taking it to 95% or more!
Re. the first point, it's a numbers thing. The efficiency loss comes from the wort left absorbed by the grain. The higher the ratio of grain to total liquor volume, the the greater the % loss of wort and hence the greater the drop in efficiency.
Re. second point, I find exactly the same. A few percent lost efficiency equates to very small extra cost that I don't think is worth the bother. I don't brew big beers but would probably sparge more often if I did.