You miss the point. I'm not just talking about diagnosibility in the field, although as far as I am aware there is no reliable way to sperate Desertas and Fea's. We can readily identify many sub-species - lots of Yellow Wags for example.Given that logic it maybe best to lump all 'jaegers' too as these are routinely misidentified on seawatches (and even when sat on reservoirs inland), then you can just tick 'Jaeger' or 'Small Skua'. One problem with this taxonomic concept is the historical geneflow between Pomarine and the 'Catharacta' group. However given that all these species are hard to identify and given ongoing geneflow it would probably be best to lump all species of larger skuas and smaller jaegers together as a single species.
The idea I'm floating is that from a listing perspective, it might make sense to re-lump certain taxa and treat them once again as sub-species. Many taxonomic decisions are arbitrary and inconsistent, and there is an undeniable bias in taxonomy towards splitting, so I'm not sure what is objectionable about creating a list that takes a more conservative approach and is useful for birders.