AdrianF,

It is obvious that it is easier to measure the molar rather then molal concentration. I agree with you. But if you want to make less errors during assays you should agree that molal approach is more suitable than the molar. With the molal approach one doesn't need to temperate all the solutions at the same temperature prior diluting to final volume (because the volume of liquids is temperature dependent, while the mass isn't).

But I also agree that the error made using the molar approach is so small that it shouldn't be taken into account for the most of the assays.