"What should be recorded" depends primarily, if not only, on who is doing the recording, and why. With the advance of recording technology, especially miniaturization, there is a wealth of recording (especially surrepititious 'live' recordings) done by private individuals simply for their own book of memories. Who's to say whether that category of recording might or might not be considered valuable by someone in the future. I have heard a goodly bit of "professional" recordings for which I wouldn't give a nickle for a truckload. But that's my nickle, my taste, and my choice. Somebody around the corner might think it's the best ever. And, just to stir the pot a bit, I think the notion of "source singers" a load of crap to begin with. In a word, it is no more than a different kind of racism.