What other reasons are you thinking of? I think the 5 listed cover basically everything. Unless you just want to be more specific?
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Although it might sound like a good idea at first, I think in general it might be a bit too difficult to make that judgement call. Many images being shared have no context, and no way to track down the original source. I think there are just too too many variables here to make an option like this work in a predictable sense., and thus would make the resulting data unable to benefit anyone.