Does that make it any less of a song? Not in my book.
It might, however, affect how much credit you give Messers Pop and Bowie for writing it.
Why shouldn't the same criteria apply to ads? Two separate questions:
1) How good is the ad? (How engaging/ entertaining/ novel/ interesting - all the normal questions we ask ourselves in assessing whether an ad is worthy of veneration or not)
2) How much credit ought to go to the creative team?
So you look at an ad like TfL's DoTheTest, you don't have to think about the fact that it's a direct rip-off. What matters is how you felt when you first saw it. In my book it's still the year's best ad so far.
I just don't think that the blokes from WCRS that did it deserve as much credit as they would if they'd come up with it themselves.
Of course, the two questions start to bleed into one another when the original source is so well known that its notoriety makes the ad feel stale.
Perhaps the moonwalking bear only worked for me because I hadn't seen the original, so DoTheTest felt fresh and novel.
It certainly feels a far less egregious rip off than either of these two, one of which lifts from the most well-known music vid of the past couple of years, the other from one of the UK's most original recent comedy series. To me, they're far more worthy of condemnation.