Well, to expound on my previous comments and to address your question “why would Miles play on a newcomer’s record?”:
Not uncommon at all. Cannonball was a rising star, not to mention member of Miles’ own band and was being heavily promoted. By some accounts it was Cannonball who received the invitation from Bluenote to record an album and that it was Cannon who then picked the band. It is not unusual for a high profile leader to play on a solo effort by one of his sidemen. Moreover, Miles was under contract with Columbia and Cannon was not under contract with any label. Why wouldn’t Columbia simply release another Miles record? Why wouldn’t Miles use his working sextet and why would he record with a different rhythm section and ask only one member of his band to play? Seems like bad politics. Leonard Feather points out that the title tune “Somethin Else” which was written by Miles is an expression of praise for Cannonball.
“He didn’t play on any of Trane’a records”
Probably because, unlike Cannonball, Trane didn’t ask him. Trane and Miles didn’t have a great personal relationship. In fact. Miles fired Trane at one point.
Lastly and probably most importantly and telling, to me, SE sounds nothing like KOB. Constantly changing musical chameleon that he was, Miles had moved to a modal style by then. KOB is the groundbreaking and best example of modal Jazz, while the material on SE is much more traditional with most of the tunes being standards which is a style much closer to home for Cannonball.
I guess we’ll never know for sure, but interesting to speculate; but that is where I put my money. No matter how the record really came about, we’re lucky that it did.
Regards.
Btw. in my previous post I mistakenly referred to Miles’ band as a quintet. Obviously, it was a sextet.