Between October 2005 and December 2007 he averaged only 30. Thats 20 matches and 38 innings. We lost 8 games and won 9 in that time. I am sure some other players we cut during that run. Of late, his performances seem to be better but I can almost guarantee that if any other player had run of 20 matches and 38 innings where they averaged only 30 as a front line batsmen they would be gone.
4 years ago? I thought you meant now? I can point out many players (not just in SA) that go through dips in form, but are not dropped. Its like someone said, you stick with your class players, even if they do go through a dip in form. Kallis took forever to show what he could really do when he started playing, and imagine where we would have been if he was not given the proper chance.
Then add the fact that Smith was captain it would have taken more of a '20 match averaging 30' run to get him dropped.
McKenzie for example, when he played years ago, he was given lots of chances and never really performed up to standard. Then he got dropped went away for quite a while, and got selected again. He then performed brilliantly for a period and then hit some bad form / bad luck. And the fact that he was at the end of his career and not a youngster just starting out with a long future ahead of him, probably made the decision to drop him a no brainer.
Some players do get protected yes, but usually not for the reasons you'd think.