It's far from ideal, but it's much less of a risk than having McLaren or Boucher come out at #6 and Paul Harris come out at #8.
You yourself said that you can't recall South Africa going with two spinners since readmission, but equally.. can you remember them going into a Test with a six-to-eight as bad as McLaren/Boucher/Harris?
Either way it's far from ideal but the sixth batsman is worth more than the fifth bowler. You can argue that a fifth bowler would be of more use than a seventh batsman, but Boucher is not a #6 batsman so that's not the debate. You of all people should be able to see that.