Atl coins that provide no new value over bitcoin, but are simple copies, IMHO are the biggest threat to bitcoin in general. I have a ready response to all the doubters questions except for the proliferation of alt coins...
I think they do offer some value as a backup for bitcoin, a distractor for authorities and people who would like to see bitcoin die, and one day a superior altcoin than bitcoin may emerge.
I understand your fear but I think that no altcoin will take away value from bitcoin as long as bitcoin remains superior. The competition is good to put the pressure on the bitcoin dev team not to screw up.