I think anyone bowling spin to SA would do a decent job in this series, just that Maxwell was the regular spinner and bowled the majority of spin to them, so the only one that can be judged with a decent sample pool. Not sure he is worth keeping in the side for 6 overs a game (his average across 4 games)
Smith took 1/17 yesterday from 5 overs (only bowled 8 overs in total)
Xavier wasn't given much of a chance 6 overs 0/31
Like Hiney has said, who are the other options? Maxwell may be in the side by default only. Unless we try a newbie like Boyce or Muirhead, or just stick with Xavier