I disagree. Like I said it's a completely different convenience. Sure, there are covers done for commercial reasons, and some work, but most fail. Britney Spears doing "I Love Rock N Roll" made me wanna shoot myself.

But most with actual talent cover songs as a tribute to those who inspired them (John Mayer) or simply because they aren't primarily writers and convey their emotions through someone else's words (Frank Sinatra/Michael Jackson). Though I guess if you wanna make logical parallels, Michael Jackson singing "She's Out of My Life" is no different then many directors who didn't write the original screenplay, but directed the final product.