That isn't true at all. SS's play a lot closer to the LOS, and are moved around a lot more. A guy like Bob Sanders is an awesome SS, but would not be as effective as a FS. Polamalu is effective because they let him move around a lot, and force the defense to account for him. This wouldn't be possible at FS. Ed Reed has amazing ball skills, and would not be as useful as a SS where he wouldn't be able to play as much centerfield.
They are very different positions in real life. In Madden, not so much. Really, until they have a true SS and FS, they should just rename it Left Saftey and Right Saftey.