While my historical knowledge of that era is not that deep, it would seem that the main reason that McKay and drummer Kenny Morris were in the band initially was that they were available, and that's about it.
Besties Ballion and Bailey recruited guitarist Marco Pirroni(later with Adam Ant)and Sid Vicious for drums(what a versatile guy!)in an ultimate expression of punk DIY for an impromptu gig. After that served its one-shot purpose, Sioux and Severin got another guitarist(who was soon dismissed), then McKay, along with Morris. Apparently there was a major falling out with those two. Budgie was brought in, I think the Pistols' Steve Jones briefly sat in, then McGeoch.
As interesting as some of the early tracks were, Juju just killed it, I wore that album out. Severin, along with Adamson, pioneered that droney, chorused/flanged bass sound that became the aural hallmark of those years, with Hook following closely.
Mid-May I saw Siouxie with a hired gun band at a festival in Pasadena, her first gig in American for a very long time, and while I know we're all getting older, her voice was in rough shape, it was weird. She tried valiantly, and I hope she gets it back. She was very entertaining otherwise, and the band was on it. She was followed by Iggy, who was still a maniac, and I was hoping she'd come back and do "The Passenger" with him, but no such luck.