Resistor is also there to reduce false triggering due to noise on the mains, which can capacitively couple either through the PCB traces, or through the TRIAC itself. Easiest to keep it there, as while the TRIAC is likely not to trigger with 4uA of current flowing into the gate, with the gate at 0.15V, it is a lot less immune to noise then, and only a tiny bit more can start the triac conducting, and with it then slowly turning on from limited gate current, the voltage across it will be high while the current is rising, leading to very high instantaneous power dissipation. This can long term damage it, as the die area closest to the gate is stressed by heating, and this degrades that area of the junction. good reason why you have a gate pull down that handles any leakage, and you drive with more than the minimum gate current.