PaulRB:
I think the adc on esp8266 was almost an afterthought, so the circuit is pretty basic. More complex adc circuits like those on AVR chips offer a choice of references (internal 1.1V, Vcc, external etc) and a multiplexer to offer multiple adc pins. The esp has only one option (internal 1V) and no multiplexer. I think if only one option can be offered, a fixed reference is preferable.
Always nice to learn something new. 