Powering an Arduino UNO through 5V
They wouldn't advise it because they provide on board regulation. By feeding it directly you're circumventing their own protection. If however, you're providing your own regulated supply there is no reason why you can't supply it directly. 

In a perfect environment, USB should already be regulated. However given the cheap imported USB hubs and plug packs that are available on the market, I would certainly argue that a good bench power supply is no problems in this regards.