Ok, so I missed this comment when it was first posted, and to me it answers what I consider to be the biggest cause of the arguments over explicit disagreements.
This is my interpretation of the above (and assuming @loarie means Identifier rather than Observer, which is often the same person):
kiwifergus
We should not be explicitly disagreeing just because we personally feel there is insufficient evidence to support a species level ID. However, if the Identifier that ID’d at species is unresponsive to a challenge over that ID, and a conversation takes place that supports bumping it back to genus, then it becomes appropriate to do so. Or perhaps just if we believe (from past conversations on similar observations) that the community would support the decision to do so, THEN it would also be appropriate to explicitly disagree.
please correct me if I am wrong