As I understand it, the rule to not split infinitives . . . err. . . not to split infinitives . . . resulted from imposing Latin on English. In Latin, it's impossible to split an infinitive, because an infinitive is one word, not two as it is in English. So it's actually an artificial rule that most of us don't follow in casual conversation. I agree that there are cases--possibly many cases--where a split infinitive sounds more natural in English than the intact version. Then again, the point you raise about adjectives being the primary splitters does suggest giving the matter thought, because many adjectives deserve to be cut anyway.