As you say, many performers do perform what is obvously the 'Steeleye version' or 'planxty version' of a song. But how exactly is this different to Martin Carthy performing the 'Seamus Ennis' version of Henry Martin compared to, say, the 'Phill Tanner' version etc...

To take this to its logical conclusion, how is this even different from Fred Jordan singing the version of "Young and Growing" that he learned from his sources, rather than the same one Walter Pardon learned from his sources. Steeleye and Planxty ARE source singers - they are my generations source for learning songs, just as I would imagine Jim Moray, Jon Boden and Tim Van Eyken will be source singers for a folk revival in 2033. It is no less authentic to learn a song from 'revival' groups because these people are/were just a link in the path that the song takes, just as Phill Tanner/Seamus Ennis/Walter Pardon were.

I feel that far too much significance is placed onto 'Source' singing and the 'original' versions of songs, much of which was Sharps doing. Yes, the routes have changed (you don't need to meet a person to learn a song from them) and the transmission process is more infalable (the accuracy of recorded performance vs the ambiguity of written music) but what you're seeing now, right in front of you is traditional music becoming an oraly transmitted form of music again, as it was for hundreds of years before Cecil Sharp... We're basicly back to where we started the 20th century. Who said tradition was dead?