In my opinion, they are always harmful. They will eventually brown out out your corals, they'll start to bleach and turn a light beige, and eventually die. Some seem more affected than others, but popular concensus is they are bad always. They are, however, avoidable and treatable, so there is no reason to add them to your tank. I DO NOT add acro to my tank without a 6 hour bath in the Interceptor solution followed by a visual inspection with a magnifying device.
I also think it's irresponsible to pass them around knowingly without saying anything. Ultimately though it's buyer beware, and you need to make an informed trade/purchase/aquisition or suffer the consequences. For me, this means always assume a coral is afflicted and treat accordingly.
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Brad
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