Wanting the child to do something is one reason. Concrete needs another. You wouldn't disagree that I'd need to call back a small child from a high traffic street or a cliff or a dangerous animal or a poisonous thing. These are immediate and obvious.
But there are still parents who handle those occasions just fine without resorting to punishments and rewards. My parents never seriously resorted to modeling my behavior though behavorism.
It may be that you lack a relationship with your kids where that's possible. Especially if you already have rules that get enforced through punishments and rewards the kid will rightly assume that a new rule that isn't enforced that way isn't serious and the don't have to follow.
I think it's very hard to talk about something like parenting because different parents have success with different strategies and most think that their own experience should be guiding for all. It's like politics in it's mind killing potential.
Followup to: Strategic ignorance and plausible deniability
My in-law always says: "For children it is easier be forgiven then to get permission."
EDIT: This post is superseeded by my Book Review: Kazdin's The Everyday Parenting Toolkit I recommend reading only that. The remaining insight of this post is: Children expend more brain power on their parents than the parents on them.
I can say from experience: That is risky.
Children (esp. small ones) expend significantly more brain power on their parents than the parents on their children (your mileage may vary). I can assure you that they will notice these cases - at least some - and take that into account one way or the other.
If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.
Possible alternative strategies:
I am influenced by The Adlerian School. Of relevance here is Striving for significance.
The testing of limits and the resulting interaction with the parent give the child a feeling of significance if the parent acknoledges the act of the child even if he doesn't agree with it. On the other hand ignoring the act of the child is negative feedback about significance.
EDIT: The asymmetry between parents and children with respect to the effectiveness of deniability can be generalized to any situation where one actor has significantly less overall information about the situation than another actor and thus might not be able to reliably estimate whether deniability is possible.
ADDED: tadamsmar pointed out that ignoring is scientifically known to be effective and the advice or rather personal expierence I have related in this post may be contraproductive (at least if applied in isolation).