I agree with you; although there's an odd tendency even with in respecatble Goethe criticism to put a lot of weight on the biography (well, Werther rather invites it, I suppose); of course, there's also the element of simple egotism. Having read an interview with the chap proposing the theory, he seems to be deriving most of his motivation from showing up professional Germanists; he sounds quite unlikable, even by the standards of conspiracy theorists. (I suppose I might be a bit biased against people who make general statements about the laziness of Germanists, though Goethe's not exactly my area...)