In Linda's defense, she didn't want to be involved in Paul's musical life. She stated several times over the years in interviews that she'd have much preferred raising the kids at home. Her minimal vocal and keyboard skills were taught to her by Paul, who wanted to get a leg up on John and Yoko as the power couple of rock (and they eventually did--John may be a saint to some rockers, but Yoko's not looked on very fondly).
As for Britney...is anyone really surprised? Seriously? Come on, you didn't think that was all her. These days you rarely hear a real live voice onstage. Those who do "sing live" more often than not have their voice corrected--there is a machine in existence that fixes one's pitch. When someone sings into it live, their voice comes out better to the audience.
And that's hardly the only thing. If they're not lip-synching to the original track (which is more common in these days of cheap management), they'll lip-synch to specially recorded tracks or mixes based on changing arrangements (witness Britney's slow intro to "Baby One More Time").
And then there are the ghost singers. Stars who have been working it on stage for a long time, dancing and singing, sounding live but not really out of breath, have another solution. Milli Vanilli took this solution to the extreme and paid dearly for it, but basically here's how it goes since we learned from that scandal. An artist will scour the earth looking for a singer who sounds close to their voice (examples: Mick Jagger; Miley Cyrus--I believe my old friend Kay Hanley of Letters to Cleo, who is known to be touring with her, is doing most of the work there, the voice sounds similar enough; Madonna; Cher, whose ghost singer used to be a Cher impersonator in Vegas and was hired fresh off the block). That person will then be hired as a "back-up vocalist," but more often than not, depending on the choreography, it will be the back-up vocalist's voice you hear singing lead. It sounds close enough to be the real artist, it's still live, they still put on the show, they still make money, no lawsuits, no bull****.
Like Meat Loaf once famously said, and like I insist to this day, the record business is nothing more than manufacturing.
"There is no problem so big that it cannot be run away from."
~ Charles M. Schulz
