SNL may not mean much anymore, but it is still a sign that you are accepted and that you have respect and are considered "cool" by the industry. Which is why, I guess, very few people off Idol have ever been invited.
Here is last season's guests...
September 24, 2011 Alec Baldwin/Radiohead (37.1, live)
October 1, 2011 Melissa McCarthy/Lady Antebellum (37.2, live)
October 8, 2011 Ben Stiller/Foster the People (37.3, live)
October 15, 2011 Anna Faris/Drake (37.4, live)
November 5, 2011 Charlie Day/Maroon 5 (37.5, live)
November 12, 2011 Emma Stone/Coldplay (37.6, live)
November 19, 2011 Jason Segel/Florence + the Machine (37.7, live)
December 3, 2011 Steve Buscemi/The Black Keys (37.8, live)
December 10, 2011 Katy Perry/Robyn (37.9, live)
December 17, 2011 Jimmy Fallon/Michael Bublé (37.10, live)
January 7, 2012 Charles Barkley/Kelly Clarkson (37.11, live)
January 14, 2012 Daniel Radcliffe/Lana Del Rey (37.12, live)
February 4, 2012 Channing Tatum/Bon Iver (37.13, live)
February 11, 2012 Zooey Deschanel/Karmin (37.14, live)
February 18, 2012 Maya Rudolph/Sleigh Bells (37.15, live)
March 3, 2012 Lindsay Lohan/Jack White (37.16, live)
March 10, 2012 Jonah Hill/The Shins (37.17, live)
April 7, 2012 Sofía Vergara/One Direction (37.18, live)
April 14, 2012 Josh Brolin/Gotye (37.19, live)
May 5, 2012 Eli Manning/Rihanna (37.20, live)
May 12, 2012 Will Ferrell/Usher (37.21, live)
May 19, 2012 Mick Jagger (37.22, live)
JMHO -- the "cool" artists (Rihanna, Drake, Usher, Radiohead, Black Keys, Bon Ivor etc.) were cool before then were on the show. And Michael Buble, Lady Anti, One Direction and Kelly Clarkson did not get "cool" for being on the show.
And the emerging artists (like Jack White) did not end up with much from being on SNL. And some of the performers should have stayed home (Robyn, Lana Del Ray) because the appearance did them no good. And most mysterious to me is Karmin -- who wasted this appearance with a meh performance (and 2 songs!!) and only managed to debut at #10 after so much hype for their album.
ETA: NO NO NO to Adam with Jimmy Iovine! Adam needs to find his own path, he does not need to be manufactured but an Iovine or Davis. I would love him to work with L.A. Reed.