The presence of the Ace of clubs may best be explained by its LACK of significance.
As a photographer I photographed many magicians. Although I tried to avoid the trite picture of man-in-tux-holding-card(s) I would not have chosen the ace of either hearts or diamonds because of the lack of color contrast in what was probably going to be reproduced in black and white. That left the choice between spades and clubs. The Ace of Spades has significance -- sinister, eerie, evil -- not what we want to potray for our magician. Ergo, we would use the poor, humble, INSIGNIFICANT ace of clubs.