Why should I use a resistor for a 3.3v-consuming led?
You are missing two things (but you are still right):


The cells may have > 1.5 V when new.
The cells have some internal resistance.


Your "circuit" is very similar to the key-fob LED flashlights which cause much puzzlement to beginners because they consist of only a coin battery and LED.



Figure 1. (left) What you see and (right) what you get.

You can get some idea of the expected current by looking at the IV curve for your LED.



Figure 2. Draw a line vertically from your battery voltage to intercept the curve for your colour of LED. Then draw horizontally to meet the vertical axis and read off the expected current. [Click for 100% resolution.]

Generally a resistor is required to limit current. You can see from most of these curves that if the voltage is a little higher than expected or if the LED curve shifts left a little due to temperature that disproportionately higher current will flow.