Tails You Lose, Heads I Win?

I'm not sure yet if it has a technical name, but one argument I have seen used to bolster one particular source theory over another seems dodgy to me: that is to assume that by finding fault with another source theory, one advances one's own source theory. For example, supporters of the Griesbach theory (Mt-Lk-Mk) seem to get excited about finding fault with the logic originally used to build the Markan hypothesis (Markan priority). But a faulty argument used to support Markan priority is not necessarily a win for the Griesbach theory.