On the later albums (starting on VII) when the band began to change Terry Kath ended up playing a lot of bass. The bass line to Dialogue and I'm a Man are amongst the first lines I ever learned. To this day I still love his playing especially on Chicago V.
I actually have a theory about Chicago. I know David Foster contributed to their downfall but I think the end of the war (Vietnam) had a lot to do with it too. They were such a strong anti-war band, and when the war ended the only thing they seemed to have left were "silly love songs". Kath's passing didn't help at all, had he stayed with us I think his name would be mentioned right there with Clapton, Santana, Hendrix ect. He was the first player in a rock band who could play over changes.