The thing with sliders is that there is no reason NOT to have them. Pretty much every interaction between two players, movement, catch, pass, kick, etc, is a formula being computed behind the scenes with a base formula and a modifier (modifier being catch, speed, accuracy, whatever). This is necessary for game developers to tweak the settings and not simply hard code numbers (this way a designer can change a variable via scripting or some other interface without actually needing a code change). Since you need these numbers in the development process anyway, why would you not make them available to your players?
Even something like speed, where you say a game should know that 70 is faster than 95 or things should be ratings based, they most certainly are. The speed parity is the crucial number here, as you most likely have some type of formula where this parity is the modifier with the player speed (rating) being the "hard" value passed in (and undoubtedly this game also uses fatigue in that formula somewhere, as I am sick of seeing tired guys absolutely gassed to the point of 5.5 40 levels in open field).
Sliders are also necessary for us to create a different experience than the one that the development team has created for us. Think how football has evolved over the years with juggernaut running teams, spread offenses, the explosion of the passing game, Wildcat, etc. With enough sliders actually working correctly, you could take a game like Madden and tweak it to play like current day NFL, or back 50 years with more defensive/running gameplans.
Sliders are a good thing (more would be even better), more documentation on their ACTUAL affect in game (even releasing the actual formulas would be awesome, seeing as EA has no real competitor in this market and nearly no one else has the resources to make/support a game like Madden EVEN WITH THE CODE BASE) is what would be really great. When you see a slider that has no effect or is not working correctly, it is most likely because that base formula is not being affected heavily enough by the modifier, or it is flat out wrong/bugged.
In short, sliders are great, guessing what they do without actually knowing? Not so great.