There are no ratings in real life. Ratings are are based on a players performance and are used to mimic that in the video game. Here's is an example of how hiding ratings would make Madden better, or more realistic: let's say a team picks up a productive corner, but the system this corner played in required him to play zone coverage a majority of the time. Assuming that he hasn't played on a team that required him to play a ton of man coverage, if you're going to ask him to do that, how would you know that he would be cut out for it ? The answer is you wouldn't ? He may be able to thrive on a defense that plays a lot of man, or he may get undressed. Hiding ratings would open up many situations like this. There's no way for a coaching staff, whose defense plays a lot of man, to anticipate how this particular player will pan out.
Potential is not always known. The draft is based on what scouts and coaches think a player's potential is. If potential was always known, Brady would've been picked number one overall, not in the sixth round. Joe Montana would have been picked early in the first round, not in the mid rounds. Farve would have been a first round draft pick and, more than likely, would have played a lot longer in Atlanta than he did.
Also, RB is probably the easiest position to project in the NFL. If a player has speed, agility and good vision, more than likely, he'll be effective in the NFL. The evaluation process isn't that easy regarding other positions.
Kevin Kolb, he performed well against the Saints, who aren't really known for their defense, and the Chiefs who are terrible. How is his awareness ? We haven't seen him against a stellar defense, or a division opponent who will see him twice a year. How is his arm strength ? Will the Eagles stretch the defense without McNabb's cannon ? How is his toughness, does he get rattled easily ? We won't know any of this until he plays more games. The way Madden's ratings are shown, we'd know everything beforehand.