You pick a team for five days of the Test match - not the first innings, and definitely not on the off chance that you win the toss and bowl first. Therefore, to go with four quicks and not include a frontline spinner is madness I reckon.
Thus, with McGain in the team, the fifth bowler, especially at Johannesburg, should probably be a seam-up merchant. = Macca.
These I think should be the two principles that guide the selectors for the first test.