Wouldn't it be fun?? I think it'd be fun. The ONE band in the universe that actually does consider the death of the singer, guitarist & songwriter to be an insurmountable obstacle, and it has to be the one I'd like to see most! xP
No reason why they shouldn't. The whole "wanna preserve the band's legacy" or "it wouldn't be the same" logic is tiresome. For me, it is more about the songs than one performer.
If I were Dave Grohl, I'd talk to Kurt's family, get their thoughts on it and if they approved, I'd look into doing something. That's just me. Hell, play a few shows for a dvd or cd. You don't even have to record new music.
For every uptight, crybaby who will complain about Nirvana insulting their sacred history, there are at least that many people who never got to see Nirvana songs performed in person (maybe they are too young) and would love the opportunity to go to a live show and hear the music, regardless of the fact that Cobain isn't here anymore.
Unless I am mistaken, I'd assume you fall into the latter group given your age on your profile and given your interest in seeing them again.
This is coming from a guy (myself) who isn't even a big Nirvana fan. There are people out there who'd like to see a full show of Nirvana songs again from something resembling the original band and someday one of the surviving people involved ought to find a way to give the fans what they want.