Steve Vai made the David Lee Roth albums a success - without a gutiarist that could top EVH nobody would have cared what DLR came out with...
Same goes for Ozzy, he was messed up and unpopular after he was booted out of Sabbath. He wouldnt have made the return he did if it wasnt for Randy Rhoads...
Not everyone may agree but I reckon Megadeth would have faded into obscurity after So Far So Good So What... if it wasnt for the injection of professionalism and insane signature sound that Marty Freidman brought. Rust In Peace is still one of the best metal albums ever made and the guitar work is incredible - without Friedman they never would have went where they did.