A session player is generally a far better musician than most rock stars, and many session players have gone on to become stars in their own right, including:
Jimmy Page - played on records by the Kinks,Joe Meek and others before joining the Yardbirds and forming Led Zeppelin
John Paul Jones - also a session player before joining Zeppelin, playing with, among others Donovan and P. J. Proby.
Glen Campbell - Played on records by Phil Spector, Nancy Sinatra, the Monkees and the Beach Boys among many others before his solo career.
Leon Russel - Another of Phil Spector's 'Wrecking Crew', also played for the Beach Boys
Mac Rebennack (Dr John) - Played for Spector, Sonny And Cher and Nancy Sinatra among others.
Luther Vandross - started out as a backing vocalist for David Bowie
Elton John - played with Long John Baldry and The Hollies

And many, many more...