I've always heard that Stewart was banished because of this incident and I'm sure it played a role in what happened to him. Plus, as you've said, why have a guy on the roster you never use? Like the Zim not using Bill Lee the final month or so of 1978 (in favor of Bobby Sprowl, among others).Sammy Stewart was never used throughout the entire playoffs. Here is the explanation per https://sabr.org/bioproj/person/sammy-stewart/
McNamara may not have cottoned to some of his clowning in the clubhouse, but Grossfeld says Stewart told him that he wound up in the doghouse due to a time the team bus pulled out and left him behind at Fenway Park. Stewart said he’d been visiting his son Colin in the hospital, but had arrived at the ballpark on time, thrown his bag on the bus, and was parking his car when the bus pulled out without waiting. He got into bitter arguments with traveling secretary Jack Rogers and with McNamara, and may have burned a bridge or two. During the World Series, Stewart said of McNamara, “He lay down on me and it cost us the World Series. I hated to see Al Nipper come out of the bullpen when I’ve never been scored on in the postseason and my arm was feeling good.” Less than three weeks later, hello Cleveland. He had prior postseason experience. He had a poor September, but to completely give up on him? Befuddling.
However, if you look at some numbers from that season, Stewart had and ERA of 1.91 on July 27. He ended up at 4.38.
In 16 outings from that point on, he gave up 2 or more runs in 10 of them. In two others, he gave up a run, none in the other 4 games. His WHIP for the year was 1.75, and clearly higher the second half of the season.
In the abstract, sure, I'd rather have had Stewart and a few other guys out there vis a vis Al Nipper, who I never liked very much.