UriGeller

Sorry for my lack of english skills.
Just before gametime, when you setting your rules up for the oppossing team. Why wouldnt you always try to make every opposing player play with his bad leg?
What is the drawbacks for setting this up for every single player on the opposing team?

No.1Gooner

Some players "weaker foot" might be say their right foot on the left wing. If this is the case, the defender invites the winger to come inside, giving him a chance to shoot/pass/cross way easier, instead of forcing him down into the corner of the pitch.

If you have a determined striker upfront it is always useful to force the opposing defender onto his weaker foot. I have found this works well to force errors.