Ronna, I don't think anyone is taking you to task, particularly for the music you are suggesting. There's no argument on this site (including from me) that the Hall has given progressive rock the short shrift since it began inducting artists. Furthermore, the Hall has made some awfully dubious inductions over the years--the Dave Clark Five and Percy Sledge are two egregious examples.
But I've had this conversation countless times over the years, and most of the time people's arguments are driven by two qualities: their emotions and their biases and limitations. Music is an intensely emotional experience--you're a musician so I don't have to tell you this--and this usually colors someone's perception: An artist you love is "great" because you love them. It's tautological (circular argument), but that's what the essence of someone's argument often boils down to. Related to this is someone's biases and limitations. Biases are the tastes and inclinations, both positive and negative, we have, and limitations are what we don't know or don't understand.
For instance, you state that "disco has never been associated with rock and roll." Tell that to the Rolling Stones ("Miss You," and in fact much of Some Girls, or "Emotional Rescue") or the Kinks ("[Wish I Could Fly Like] Superman"). Boz Scaggs is primarily associated with the disco era ("Lido Shuffle," "Lowdown"), but he started as Steve Miller's frontman in the 1960s (when Miller's material was more progressive than his better-known 1970s hits), and Scaggs's first album, cut with Muscle Shoals sessioneers including Duane Allman, is a blues-rock gem.
Based on your comments, you appear to be passionate about this subject, which is great. However, your definition of "rock and roll" may not match someone else's definition of "rock and roll" that includes disco. And who knows? That person's definition may not include progressive rock. And in my definition, either omission is a shame because both genres help to define the Western popular music the Hall has been recognizing since 1986.
By the way, I doubt Small Faces would have been inducted were they not attached to Faces (and vice versa). And, yes, Earth, Wind, and Fire is in the Hall--selected in 1999, inducted in 2000. Even if "September" and "Boogie Wonderland" are prime disco tracks . . .
Kiss (a band that calls themselves pure Rock and Roll) put out their Disco album in 1979. Nobody can tell me that "I Was Made For Loving You" was not strobe light inspired.
DDT, going back to your musical apartheid reference, you are bang on. I think in the I-Pod shuffle generation, and the lack of importance of the album, the newer generation (and damn I feel old typing this) are not really growing up with radio like we did, and feel subject to the same musical categories that we did. I remember feeling strangely guilty for loving Public Enemy as a white kid with his jean jacket and longish hair in the late 80's. Because I didn't look like someone who knew the words to "Fight the Power" doesn't mean that I couldn't recite it on command, and appreciate the message; even if it did not apply to me.
Conversely, my closest friend (who if Jamaican-Canadian) told me his love of Hall & Oates; (who may be white soul, but looks like white bread...especially Oates) and my first reaction was disbelief. After, I though how foolish that was, as I made a musical racial generalization....on someone I have been good friends with for two decades........but he doesn't know who the Allman Brothers is...