I thought the nadir had been reached when punk godfather Iggy Pop's celebratory ode to hedonism, "Lust for Life," appeared in an ad for a cruise line: It couldn't get any worse. Now along comes a commercial for AARP, the American Association of Retired Persons, showing people "of a certain age" frolicking to "Everybody's Happy Nowadays" by English '70s punks the Buzzcocks.

The debate over rock musicians selling songs to Madison Avenue is an old one -- it started long before the Jefferson Airplane did a blue jeans commercial during the Summer of Love -- but the practice is now so widespread that few advertisers even bother to write their own jingles, as in the days of indelible ditties such as "Plop, plop, fizz, fizz ..." and "I wish I was an Oscar Meyer wiener." Why should they, when so many good songs are available and so many artists are willing to sell them?

NPR's "Morning Edition" recently examined the phenomenon in a two-part story that quoted a variety of musicians offering the usual justifications. One can be dubbed "the Moby defense," in honor of the techno-pop guru whose brilliant 1999 album "Play" was ignored by radio and MTV until the songs got airplay on TV commercials. Bachman-Turner Overdrive vet Randy Bachman told NPR he made more money selling "Takin' Care of Business" to an office supply chain than he did from his '70s recordings -- a dubious claim, since licensing fees average in the low five figures -- and indie-rockers the Spinto Band said that the money they made mortgaging "Oh Mandy" to a department store funded a European tour they otherwise couldn't have afforded.

I'm no communist; I believe musicians should be paid for their work, and well enough to afford luxuries like health insurance. Of course their art is inextricably tied to commerce, unless the only way they disseminate their music is by playing for free in the park. But I also believe there's a line where the commerce conquers and kills the art.

Never again will I hear "Lust for Life" in quite the same way, grinning at the cheerful perversity of the line "Of course, I've had it in the ear before"; now, I'll always flash on a smiling family playing shuffleboard. Granted, my anachronistic art-critic objection that commercials rob viewers of the power of imagination can be countered by postmodern theorists who insist that media-saturated, digitally savvy Generation Y doesn't live by such old-fashioned rules -- that it's perfectly capable of grooving on Iggy without thinking of cruises. So let's set that argument aside and look at the other two reasons why commercials corrupt.

Following my negative review of John Mellencamp's new album "Freedom's Road," his publicist charged that I missed the irony of songs like "The Americans" and lyrics such as "I try to understand all the cultures of this world / I'm an American from the Midwest." Like Randy Newman, Mellencamp is supposedly protesting that our nation isn't more open-minded. But the Hoosier auteur sacrificed any claim to irony when he compromised his longstanding pledge and allowed "Our Country" to power a simplistic, flag-waving, decidedly un-ironic car commercial. You can't have it both ways, John.

On the other hand, the Lips sold the same song to an unimaginative car commercial that only cheapened the meaning of "Do You Realize??" as an existential anthem inspired by the death of Coyne's father. The band's manager admitted that commercial was a mistake -- that it had trusted the ad execs to do a good job, but were sadly disappointed.

And this cuts to the heart of the problem.

No matter what the genre -- pop-punk or hip-hop, classic rock or dance-pop -- the best bands work long and hard to create a complete identity, considering every aspect of how their songs are written, recorded, packaged and sold. When they sell their music to a commercial, they cede control to an advertising director, and they take their chances.

I've never interviewed a musician who doesn't want people to buy his or her music, but I've never talked to one who doesn't also have the goal of moving listeners -- making us dance or laugh, cry or smile, shout in anger or jump for joy. Advertisers have one ambition: They want you to buy their product. If a band cares so little about what becomes if its music that it's willing to sacrifice an emotional response in favor of a commercial one, why should we consider their music as anything other than one more disposable product?