I agree the the CV training data selection does a good job sampling from all available photos.
However, I was referring to the utility of targeting observations of taxa to add observations so that they can be included in the CV. if just one user is doing this for a species with very few observations already, their photos may make up the vast bulk of a training data set.
See an example of the issue here: https://forum.inaturalist.org/t/how-are-photos-selected-for-cv-training/42403/2