In the case of the Stones, I think a better example to point to is Darryl Jones, who's been with them since 1994. Ron Wood was admitted as an "official" member right from the beginning when Mick Taylor quit. Jones, on the other hand, still hasn't been, which is why the "official" 5 member band has been reduced to 4 since Bill Wyman left. In this case, I think it may have a lot to do with "image," where the big burly black American musician doesn't "look" like a Rolling Stone whereas Wood does with his pasty complexion, slim rock star physique and spiky long hair. Going way back to the beginning, their original manager determined that founding member, Ian Stewart, didn't fit the image and was thereby relegated to roles outside the photo frame. I imagine Steve Jordan will experience the same fate, as has other touring members of the band over the years (e.g. Chuck Leavell), where now it may be just as much a matter of compensation and "share" of the band's profits as one of image. Besides, at this point after more than 50 years, new members wouldn't be accepted as "real" members by fans anyway.