They were never the darlings of critics, but neither were Black Sabbath when they were operative and look how they are perceived today. And there is no denying that in the first half of the 70ies, GFR were a very major band on the US circuit with the record sales and the airplay to show for it. Terry Knight's heavy-handed svengali/mobster type of aggressive management did their legacy no favors in the end (nor Farner's evangelical fervor later in his life), but the earthiness of their sound and their apparent Detroit Music/Motown influence set them apart from being just another Cream-derivative hard rock outfit. There was some substance there if you listened closely. In my ears, they progressed musically noticeably when Craig Frost joined them on keyboards and they turned into a four-piece, I like those later albums when they were on a commercial decline.
They are certainly more musically deserving than Kiss (who nevertheless deserve their place in the RRHoF as a cultural, commercial and live performance phenomenon).