It seems like you are heading down the path where black hat anti-SEO on competitors becomes an increasingly viable business model. For example, company A competes with company B. They are cutt-throat so they pay a blackhat SEO firm to generate obviously spammy links for their competitor, not in an effort to bring up their own ranking, but to tank the ranking of their competitor relative to their own. At the end of the day, you do not know the intent of a link. Discounting a low quality link is reasonable. Penalizing the site it points to is not.