One thing I've noticed from doing renderings is that there should be billions of combinations with the seed, and yet it's not rare for the mistletoe to reuse a seed when set to -1. I noticed this with identical or very close requests, while if it really took a random seed these coincidences should not be possible...
If he does this, then there is a chance that several people with the same ideas will end up with quite similar images because the chosen seed isn't random enough.