Players may have common abilities, but other attributes will vary. Sayers and Simpson could both have speed burner abilities, but one may have a higher agility rating than the other, one may have higher breaking tackles than the other, one may have higher acceleration than the other thus giving them the different feels despite sharing certain abilities.
The only thing that really makes a legend a legend is having one or more abilities they excel in that generics can never have.
You could have a generic player with ratings that make him on par with the "speed burner" ability but his catch rating is 60 or less. His route running is not as high so, when we play with him we go damn he can out run the Legend Terry McDaniel but the sucker cant catch.
I think they did a great job at making players vary enough that you can choose players to fit the scheme you are trying to run without making any one squad overpowered. Even if you have a whole defense or offense gull of legends, they still have weaknesses that a generic can possibly overcome which balances the game.