So what you're saying is Adrian Wilson is a poor man's Roman Harper? Roman Harper isn't even a poor man's Roman Harper. I have a hard time believing there is a safety in the league that is worse than Harper in coverage.
But on a serious note, most of the older/veteran SS's arent great in coverage and are looked at as more of the "in-the-box' type. Its the younger guys that are coming into the league that are more over tweeners who can cover and play the run.