If the transistor is not fully saturated it will be acting like a variable resistor instead of a full on switch and may overheat, also, if the relay coil is not getting enough hold in current, it could drop out with a hard mechanical shock, I wouldn't use the second R, nor second guess the module designer.
Hi guys!Thank you both! The relay was working fine, drawing roughly 3.5 mA from the ESP.I was curious to try PieterP's suggestion and add a second 150 ohm resistor in series with the module.It still works great, and current has gone down to 1.65mA!So I guess I'll stick with the resistor. Any disadvantages? (apart from extra cost and breadboard space?)Thanks!Marc