Like I said it's about balance, that example would obviously be too far the other way. Portishead, who headlined the other, were close to pure art and however utterly sublime that set was it wouldn't have worked the same on the Pyramid as headliner.
Imagine the history of Glastonbury if they stuck on whoever was at the top of the charts as headliner rather than factoring in artistic quality. Glastonbury wouldn't be what it is today if they did.
For Glastonbury headliners, artistic quality shouldn't be optional, it should be the central question.