While such a classification is desirable for petrologists, the average earth scientist relies on a much simpler scheme. That classification takes advantage of simple associations that occur among the various silicate minerals. We do not need to know percentages of the various mineral phases, merely which minerals are present. While not as accurate or precise as the IUGS classification if is more than adequate for field and lab studies. We will be utilizing this classification as our basis for identifying igneous rocks.
Volcanic rocks present a greater challenge.
Since many of the mineral grains are not visible, using a mineralogical classification becomes problematic. Ideally we would like to have a chemical analysis. However, most lay people have little access to analytic facilities and a classification based on chemistry, although desirable, is rather impractical. Thus, most field classifications of volcanic rocks rely on the few phenocrysts we can see or the rock's color. The latter can be especially unreliable, but often it is the only clue me have.