As a child of '80s hair metal, I often thought of rhythm guitarists as failed lead guitarists. While there were exceptions, I'm pretty sure that was true in more cases than not. It's not true in the blues. There are an awful lot of guys who made a name for themselves in that role of rhythm/second guitarist. Some rhythm guitarists were practically playing a second lead line that complemented and competed with the other instrumentalists. Some rhythm guys were crafty, finding small cracks in the arrangement where they could add some magic. Some of them were lead guitarists just looking for a pay check backing up a friend or colleague.
John Primer played in Muddy Waters' band, Willie Dixon's band, and then spent more than a decade as the rhythm guitarist for Magic Slim & The Teardrops. Listen to "I Called My Baby" and tell him he's a failed lead guitarist. Listen to it and tell him he's a failed anything.
Primer leads his band through "I Called My Baby" and it sounds almost like a posh, upscale J.B. Hutto & The Hawks performance and tune. Primer plays a nice slide lead throughout the track and it has the the sharp sound Hutto and Elmore James used to create so brilliantly but Primer's touch is gentler and more nuanced than the gutbucket, slash-and-burn approach favored by Hutto. James and Hutto both had larger-than-life vocal styles that emphasized raw power over diction. Primer's voice has authority and just a little grit, but he's not out there killing it.
On Hutto's Hawk Squat record he had Sunnyland Slim there to play piano and organ, a role filled wonderfully by David Ross. Melvin Hinds' harmonica quivers in the background like some classic James Cotton, who I don't think ever played with Hutto but it sure sounds good here and has to be mentioned.