Yet I've heard people suggest that you must never be dismissive of things said by smart people, or that the purportedly high IQ of the LessWrong community means people here don't make bad arguments.

When? The closest case I can recall came from someone defending religion or theology - which brought roughly the response you'd expect - and even that was a weaker claim.

If you mean people saying you should try to slightly adjust your probabilities upon meeting intelligent and somewhat rational disagreement, this seems clearly true. Worst case scenario, you waste some time putting a refutation together (coughWLC).