Oh, I think quite a great deal of harm, if reason is something to be valued. Most pointedly, his defence of the Heidegger against the charge of Nazism is a pretty serious abuse of Derrida's authority. More generally, Derrida's elevation of marginalia, word-play and other aspects of the frivolous have had a desultory influence upon scholarship.
It seems a shame that those on the left who hear the likes of Roger Scruton defending reason draw the absurd conclusion that reason is right wing, and obscurantism and irrationalism is left wing. After all, it is the left that has most to lose in the degradation of rational investigation.
As a philosopher Derrida presents an interesting paradox (as do other skilful philosophers). Seen in the round, his work is part of a tendency towards stupidity. But his specific technical steps of argumentation are intelligently crafted. I think it should, after Hegel, be called 'the cunning of unreason' that it advances its cause through well-woven disputation. -- Jim heartfield