Travis Day talks about optional challenge through Monster Power in this post, and (for reference) here is the Itemization blog that mentions the future of Legendary items. I think it’s difficult to anticipate how those future updates to Legendary items will affect the challenge in higher Monster Power settings without knowing what other updates might be made to the game, but viable optional challenge is important to Diablo III. If players find those higher settings too easy, then we will be sure to let the developers know.