I think political/social eras are useful as well. Most of Marvel's 60s superhero comics can accurately be characterized as "Cold War comics". An most Bronze Age comics can usefully be lumped under the category "70s Malaise era comics".
Yup, I agree with you there, and it's a sort of nomenclature that can only be added after the fact. Much like Shakespeare and his lot weren't going around saying "We are making Elizabethan literature", nobody ever thought of themselves as a "Bronze Age Superman writer". As you say, it's far more useful to put them in their political/social context: The Golden Age is basically the end of the Great Depression and WW2, the Silver Age is the Cold War and to an extent the Swingin' Sixties (though for obvious reasons the counterculture was better represented by underground comix), then we move into 70s malaise, or Vietnam and Opec as I like to think of it, and so on.
Divisions like this are useful as shorthand in discussions, but are essentially artificial. I'm put in mind of the comedy sketch - can't remember who did it - in which a man walks onstage and says: "Hey everybody! The Thirty Years War has begun!"