Who Knows for sure. Like you said, probably only the hard-core players would customize every player on ever team, every teams colors, stadiums, names, etc. and there are probably some that would do very little if any editing, while the rest are probably somewhere inbetween. As long as it's not too costly, it seems like your always better off giving the user more choices....at any route, all they have to do is make everything generic and let the user do what he wants. He can edit his team or every team or leave it be. He's not losing out if he doesn't want to edit, he's just not taking advatange of what the game offers.
And hell, if the game still wants to use legends to sell the game, then still include them in the game and allow the user to choose whether or not he wants to include them in his rosters. Similar how NCAA allows you to randomly assign names to each player, you could have the computer randomly assign legends to teams if that is the way you want to go.