Are praying mantises helpful or harmful to people and why?

A praying mantis cannot harm humans outside of small bites and scratches, but as they are not poisonous, these injuries don't hurt any more than an ant bite or rug burn. They are predators, and eat other insects, so in that regard they are helpful; while it is unlikely that they would get the opportunity to snack on a mosquito, they certainly would given a chance. They are more helpful than harmful, but in point of fact are really . . . neutral.