Few things, first, it's not a hologram like you see in star wars, it's a projection from a series of mirrors and glass, like you'd see in Scooby Doo. Secondly, the projection of other dead musicians would give a really dry feel to concerts. Since they're just projections, interactions are limited. So seeing someone like Jimi or Cobain (who'd probably be rolling in his grave at the thought of it all, ) they'd be set, no improvs or 10 minute jams. However, you can probably project Van Morrison, and no one would be the wiser.