Well it's not so much just my opinion but based on what an "era" is actually defined as.
Definition of era
1 a : a fixed point in time from which a series of years is reckoned
I believe that's exactly what was being cited when referring to "The Hackett Era". The time and years he was in the band. Not what he did or didn't do, how popular or famous he was, or how big or little his contribution was but just simply the years he was a member.
Leaving aside the lesson in English (from an American, yet!) I maintain that to name an era after someone who had so little to do with the writing and the arrangements just seems perverse. You seem to have a bee in your bonnet about fame and stature; given that I mentioned neither, I can't help but wonder about the nature of your fixation.