Well I'm sure most bands that get back together (I.E. weezer, the pixies, etc) don't TRY to not top their last albums... but sometimes the magic isn't there, or maybe you're connected through nostalgia you know?
I often think that's why I like Pinkerton so much.. Nostalgia, not Weezer..
Who knows.
I agree, most of the times though you're looking for progression in the band. That's how it used to be in the 60s, bands would progress in their music through out the years, nowadays it seems to be the opposite.