Doing so relies upon the FET not passing the voltage back to the USB and thus the computer (two 5v sources connected).
The Arduino +5vdc shield pin and the USB +5vdc are wired to the same side of the FET, so it offers no isolation between those two. So wiring an external regulated +5vdc to the +5vdc pin does result in wiring the USB power source (if/when pulled into a PC) and the external power source together. This is not a recommended good engineering practice, however many have done it without reporting any damage or problems. When I wish to power my Arduino board from an external regulated +5vdc, I use a hacked usb cable and supply the voltage via the on board usb connector. This of course limits the current capacity to the 500ma thermofuse on the Arduino, so any external loads requiring addition +5vdc current must utilize additional wiring to the external power supply.