My brothers 96 cherokee was intermitently running shitty shortly after I changed the pre-cat o2 sensor. It had thrown a soft code for downstream o2 sensor. I asked around and some people were saying that it does affect air fuel mixture, while others said because the primary was so new and secondary was so old that that could cause the rough running, didn't make sense to me but I changed the downstream sensor and the problem went away.
But ya I've always thought the primary was there to adjust air fuel while the secondary was there to verify the cats doing its job...
'96 Z71,350 with boltons, built 4l60e, 14 bolt, 6"+3" body, AAL, Posi-Lok, 37s