It really depends on the style you're trying to convey. A realistic sports simulation should not use cel shading, rather they should have realistic looking athletes and stadiums, ect as opposed to having a cartoon look. By contrast, if you make a game based on a piece of animation, then cel shading and making the characters look true to the animation they are from is really the way to go. It would be a bit like making a new Wing Commander game and rather than have live actors or realistic computer generated models look like your wingmen from the Japanese instruction book for the original Wing Commander. (If you don't know, they are drawn in the traditional manga style and only resemble the characters in game. Go through some of the old posts and you'll find them). It's a style that many probably wouldn't think would work. The same as if they made Disney characters look like real mice, real ducks, real dogs, as opposed to their original cartoon look. There are cases where you could get away with doing one style as opposed to another, but you really have to try and go with the grind of what the style of game is. A cartoon world or one that really doesn't set a serious tone, then sure, go with cel shading, but a realistic combat simulation that is intended to convey the seriousness of war, then you probably wouldn't. A few cel shaded works that are serious in tone work very well, such as Jinroh or Batman animated, but again you have to try and work within the realms of what style of game you are making.