A person who prays for someone else in a serious way has to make an effort; he/she devotes time to the other person and has to concentrate and focus on that person. A serious prayer requires dedication and compassion for another person.
And what does that actually achieve?
Or another interesting question: What happens when lots of people pray for opposing outcomes in a certain matter? Does God go with the majority opinion?
If God is all-knowing and all-powerful, and has decided that a little school boy should have deadly cancer....will a large group of people praying change God's mind?
(Says the half-serious Catholic...)