You have to look at the band in the context of the time they were performing in. Nirvana is not "great" musically, by any stretch of the imagination, but time and place, man. They were relevant - and can continue to BE relevant - but you have consider what was going on in the world.
And people ALWAYS feel more strongly about those brilliant sparks extinguished before their time. (Joplin, Hendrix, Cobain, Lennon, Morrison, et al.)
The Beatles might be overrated, musically, but that's not all they represent. Just like Elvis.