I think what will be more likely is the other direction. The monitor powers the laptop. Most laptops are well under a hundred watts and could easily be charged and powered that way.
Which is a great new other feature of USB Power Delivery: peripherals can provide bus power. This way you might have an external USB monitor (USB A/V you were announced 2011: where the _hell_ are you?!) that also provides power.
For a while it seemed like DisplayPort or DockPort might be the one cable to rule them all... USB PD seriously changes the footing of the game. Alas, USB A/V (encoded video to an output device) being completely absent is seriously damning USB's chance to be that one cable.
Experience dictates that about 95% of manufacturers will be too cheap to implement that bus power on their devices, because it'd cost them 50 cents more. So it won't matter much.
