Tag 'duo' seems useless

I don’t see the point of this tag and why anyone would search for it or use it in a filter. I mean we are not danbooru where even the count of chars in a picture get tagged.

Well for instance, “straight, duo” would be quite a deal for finding stuff that isn’t, and I gotta quote @LightningBolt here, “(…) just an excuse to have a female with an anonymous dick harassing her various orifices in different ways”.
 
The distinction of group dynamics between two and more is nearly as remarkable as one and two IMO, especially in non-sexual interaction.