Not true. You can have ancillary modules, the obvious example being the SCB but in my case also the KWS. They aren't on all the time, but when they are on they need to take power in preference to other modules. You use power priorities to manage these modules without shutting your shield generator down every time you want to scan something.
Edit: to give a concrete example,
here is my current loadout. Not the type of loadout you're going to be able to pull off without putting a bit of thought in to managing power priorities in more situations than a degraded power plant.
