But if we apply the same rigour to the case where the sensor is simply powered from the same 3v3 line as the mpu, might not additional circuitry be called for there too?
Yes, to a lesser degree though.
The noise coupling to the IO structure is not well defined. Noise may be low, or may not, and this may depend on the actual IO port and pin - how it happens to route inside the chip.
Note that the ripple comes from the internal logic (CPU, memory cells, etc.) switching. This high-current loop is shared by the supply that goes to the IO pin, so:
+Vcc -- RL -- IO pin -- RL -- core
is worse than
+Vcc -- RL -- IO pin
+Vcc -- RL -- core
(where RL denotes the R+Lparasitics of the internal wiring.)
, because the core current pulses affect the IO pin voltage in the first case, but not in the latter. (Assuming there is a good bypass capacitor at the +Vcc pin.)
Same happens in a poorly designed PCB, and people still usually get away with these!
But if you design the PCB properly, it's like the latter case:
+3V3 --- RL --- (bypass cap to GND), MCU Vcc
+3V3 --- RL --- (bypass cap to GND), Sensor Vcc
(Global bypass cap at +3V3 node.)
Yes, if the analog performance is of utmost importance, I wouldn't consider powering the device from the GPIO pin. Not only due to noise, but for precision work, you likely want stable, locally regulated voltage as well, because most precision devices don't have infinitely good Power Supply Rejection Ratio, and precision analog devices tend to be rated for a certain recommended operating voltage.
But do note that the typical 100nF capacitor
exactly takes care about this high-freq noise resulting from gates switching. The IO port MOS resistance actually forms an RC filter in this combination.
And the typical 100nF cap doesn't cause too much loading for the port during switching. The peak current exceeds the MCU IO port ratings, sure, but it's a very short pulse and total energy dissipated shouldn't be enough to cause localized die heating, or electromigration. I would be far more careful with 10uF, though.