Put realism where it is best suited. For instance: take the recent dead rising 2. If the game were realistic: One zombie bite = infection. However both the gamers and devs realized how much this would limit the gameplay and decided to overlook that small detail.
However, I wouldnt be against a realistic zombie survival game. Using tools, skills, and an involved inventory to survive in a wider and more varied world, where one zombie bite could end it all, and you have to use your wits, rather than a motorcycle straped with dual chainsaws to survive.
Both sides of the coin, realism and fantasy, are fun if done right.