Okay, here's my speculation:
I think the legend has been overstated. Musically, Mick, John, Danny, and Christine could have toured as a four piece. Christine did it with Chicken Shack for years, and the Mac, to my ears, only used two guitarists at one time when they had three guitar players. So, being a quartet wouldn't have been so unusual.
However, I don't think Danny and Christine had enough material ready to go on as a four piece. Neither had a lot of non-blues material out at the time, and the band was trying to break away from the blues. What they needed more than an extra musician was a way to fill time. So, they asked Peter, and he agreed on the condition that HIS contributions to the set would be jams based around "Black Magic Woman."
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On and on it will always be, the rhythm, rhyme, and harmony.
THE Stephen Hopkins
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