On the other hand, it's good to set goals, and for kids especially it's good to have a concrete way to visualize those goals. It's hard for a preschooler (who can't easily count past 10) to understand what it means to do something "every day for 2 weeks." (What the heck is a week?)
I agree that tying it to an external reward can be risky. It might be better to do the chart without any reward. Or make sure the kid chooses an activity as the reward, so it's more like "rewarding myself" instead of "earning goodies".
Heck, it's useful for adults too - lots of popular productivity techniques are basically grown-up star charts, for example: http://lifehacker.com/281626/jerry-seinfelds-productivity-se...
I agree that tying it to an external reward can be risky. It might be better to do the chart without any reward. Or make sure the kid chooses an activity as the reward, so it's more like "rewarding myself" instead of "earning goodies".