Last week the National Day of Prayer was observed. It is a very curious concept, if you look at it logically (ignoring the initial hurdle of church/state separation). In general, prayer is either a "thank-you!" or a "request." If it is a thank-you, then the only logic issue is that a person will constantly give thanks no matter what befalls him or her, which does not occur in any other aspect of life (i.e. you don't thank a person who shoots you for fun). On "requests," if a deity exists somewhere -- and we assume this deity is all-loving, all-knowing and all-powerful -- won't the deity always know the absolute best course of action to take? Won't the deity always follow that most perfect course? Many people refer to this as their God's "plan."
If any request is made of this deity, wouldn't that request either already be part of the perfect plan or not? To imply that the deity would change its course is to imply that the deity was not already planning to follow the most perfect course, and your suggestion could make his future course of actions better.
Following either option down the logic path is an interesting thought exercise; I wonder if people will take a moment and think this through.