If you're measuring a whole chicken breast by weight, what are you going to do if a piece exceeds the weight called for in the recipe? Are you just going to cut off the excess bits? And where would those excess bits go? The bin? The freezer, perhaps, but which recipe would call for "just a sliver of chicken"? Would you rather not just keep that excess weight in and have a bit more chicken in your meal just to avoid all the fuss?