The BSC is the most common concept that defines species, although genetic analysis and the resultant data increasing play a role in drawing species lines. Genetic studies of these two warblers also do not bolster the case for separate species. They are 99.97% genetically similar. But morphology has apparently won the day, and the two are retained as separate species based primarily on their very different appearances.
WAY back in 1835, John James Audubon speculated that Blue-winged and Golden-winged warblers were the same species. Indeed, it seems that they should be considered forms or subspecies of the same entity. It's possible a bit of politics enters into the decision to retain them as separate species. Can you imagine the hue and cry in the birding community if these two "species" were lumped into one?
Independent of the human obsession with sorting and pigeonholing organisms, and the problems sometimes encountered when doing so, I will continue to enjoy the unfolding drama of the Blue-winged/Golden-winged clash in northern Michigan. Personally, I do not see this as a conservation issue - certainly nothing approaching the true conservation concerns surrounding the nearby breeding Kirtland's Warbler. Rather it is a case of long-term (or maybe short-term!) evolution playing out its hand. The Blue-winged form holds primacy over the Golden-winged form, and who are we to say that this is bad? I suspect Nature knows better than we as to what this "species" needs to expand and flourish.