Could possibly be a waste of power depending on the design. If you replaced all outlets with something like this that may have another transformer plus the power regulation circuitry your outlets would be burning power 24/7 (not just when you use it).
At least with an adapter you may only have one and unplug it when its not needed.
"Please also note that the USB ports only draw power when something is physically connected to the port. We didn't want a vampire port that continually sucks and wastes power when not in use so this was one of the features on the top of our priority list during the design phase."
At least with an adapter you may only have one and unplug it when its not needed.