Levine thinks fans should stop expecting everything from Spears and her ilk.
"No one is going to go see Britney because she's a great musician," he said. "She's a good singer, but she's a personality. She's a phenomenon. She's not an artist, she's a pop star.".
But Tyler argued that Spears could up the ante, saying her "dancing is pretty physical but it's not so grueling that she shouldn't be able to sing as well."
Unlike Spears, Madonna's routine on her Sticky & Sweet world jaunt and on tours past is more suited to marathon runners than musicians. The 50-year-old queen of pop performs high kicks and running jumps as she bounds across stage, yet never seems to struggle for breath. It's a tell-tale sign she's probably performing with a backup voice track.
"If you look at the way pop stars perform, the way they're twirling, running, being carried, dancing on the stripper pole upside down -- they can't possibly be singing every word of every verse," said Levine. "A lot of times it's blended, they might be singing with a safety net. Any pop singer putting on a full dancing show is not singing every word in those songs. The fact that they're not breathing hard should be enough to clue you in."
At London's annual Q Awards show in 2004, Elton John bashed the pop star for not performing up to par on her Re-Invention tour, saying "Madonna, best f-- live act? F-- off. Since when has lip-syncing been live? Anyone who lip-syncs in public on stage when you pay [about $169 per ticket] to see them should be shot."
Liz Rosenberg, the Material Girl's publicist, shot back, "Madonna does not lip-sync, nor does she spend her time trashing other artists. She sang every note of her Re-Invention tour live and is not ashamed that she was paid well for her hard work."
The debate didn't stop there. In an attempt to help out his pop star friend, actor Rupert Everett added fuel to the fire, telling the British press, "Madonna sings everything she can sing. ... But, if she goes into a dance routine, she's got to dance. You can't breathe and dance and sing at the same time."
Some stars couldn't cry libel or count on their celeb friends for defense when it comes to lip-syncing accusations.
Ashlee Simpson, the pop-punk younger sister of Jessica Simpson, learned that after her headline-making October 2004 blunder on "Saturday Night Live."
Attempting to perform "Autobiography," her second song of the night, Simpson found herself singing along to the music and lead vocals of her first song "Pieces of Me."
She held the microphone at her side as her canned voice boomed across the studio, danced an awkward jig and then ran off the stage as the vocal recording was shut off and NBC cut away to a commercial.
At the end of the show, "SNL" host of the night Jude Law quipped, "What can I say? Live TV." Simpson, standing next to him, blamed the mishap on her musicians. "My band started playing the wrong song," she said. "I didn't know what to do so I thought I'd do a hoedown."
A statement issued by Geffen Records, Simpson's label, claimed there was "a computer glitch," while a representative for the show said that the song that came up was a backing track.
The following Monday, Simpson called into MTV's "Total Request Live" and explained that because of complications arising from "severe" acid reflux she had lost her voice and that her doctor had advised her not to sing.