I've always thought there would be an opportunity for certain bands/musical acts to perform at SDCC and be a huge hit with the crowd. But I have yet to see it.
Has this been done? It's hasn't (at least in any significant way) at the ones I've been to over the last three years. I'm thinking theatrical metal bands would kill.
* Dethklok in 2012 played as part of a free [adult swim] even at the Midway (with free booze no less). That was pretty amazing, and that night is in my Top 3 of favorite Comic-Con memories/events in the 16+ years of attending Comic-Con
* Tenacious D played a full 60+ minute set in Hall H in 2005 to close out Saturday. Jack Black was in town/Hall H promoting the upcoming KING KONG (which was the panel before The D), and Tenacious D had THE PICK OF DESTINY. The set was great, the band was entertaining, and they played several songs off their upcoming album (lip-syncing "Mast Exploder" in such a way that my stomach actually hurt from laughing so hard)
* Barenaked Ladies did a very short one-song performance at the Big Bang Theory panel one year when they were in town (coincidentally) the same day as the panel. They just played the theme song to kick things off, but they were a HUGE hit.
* GWAR has played sets nearby, and have had members of the band at booths (in costume) doing various things
* Metallica a few years back, as already mentioned
* Metric played post-SCOTT PILGRIM vs THE WORLD premier screening in 2010 (Metric played "Black Sheep," song played by the fictional band Clash at Demonhead), with a pre-show set by DJ Kid Koala and Dan the Automator. All musical sets were awesome (as is the movie, which I saw twice at Comic-Con that week!); Metric didn't play the other nights of screenings, that I know of, but I think Kid Koala opened all three screenings with a pre-show set.
* Slayer this year on Thursday night at House of Blues (my excitement for this is well documented in the Slayer thread)
* I think Aquabats played in 2014; I remember seeing their tour bus in the parking lot behind the Convention Center that Friday (when I was in line for Hall H Saturday)
Those are just ones I can remember/attended. I'm sure there are a plethora more I can't remember. I agree that these types of events seem to be both wildly popular & fund, special-felling events for the fans. I wonder if this type of thing will become more popular; it kinda seems like it's already more popular now than just a few years ago