I think it probably has something to do with the fact that Cream (Ginger Baker) and The Jimi Hendrix Experience (Mitch Mitchell) were the 2 big power-trio's in 1967/68 - when Brian posted on the Imperial College board.
So they (Brian and Tim Staffell) were looking for someone who could play like the 2 of them. And given that Smile was a 3-piece originally (give or take a keyboard player here and there) then it was all the more fitting.
Ginger was a big influence on Roger, however, a bigger one was John Bonham of Led Zep. I think he has traits of both of them in his playing, it is more noticeable with Bonham, even Roger's kit is basically the same set up, just bigger. With Ginger Baker I think it was more about some of the subtle elements of his playing, which do appear more apparent in the early Queen recordings.