My sense of why pro brewers go by % extract is that they are brewing their recipes by extract, not by weight. The breweries have the luxury of seeing the malt analysis sheets for all the malts they use (or maybe even doing their own analysis of each lot), which allows them to alter the amount of malt used based upon the amount of extract expected per pound for that lot of grain. In terms of consistency, which is what pro brewing is about, brewing by extract is more consistent.
For us homebrewers, going by weight and accepting that +/- a couple of percent difference in extract between similar malts is just fine.