You really need to use your turn signal if you are turning left at a light without a left-turn lane, and you need to do so as far in advance as you can without being confusing to other drivers.
Using your turn signal in this situation allows the drivers behind you to position themselves to pass you on the right as you wait for the light to turn yellow again so you can screach out of the intersection and on your merry way.
I don’t care if you neglect to use your signal in a turn-only lane. I don’t mind much if you safely change lanes without signalling. It’s even forgiveable if you turn right without signaling, making me decelerate slightly behind you and wasting my valuable momentum.
But turning left without signalling in a cirumstance wherein using your blinker would have afforded myself and other drivers the chance to make it through the light by pulling around you, that is a cardinal sin of driving.
p.s. Everyone makes mistakes. If you find yourself needing to turn left, and you have not had your signal on, it’s OK. Simply suck it up, drive straight through the intersection, and drive around the block. Making half a dozen cars behind you stop and wait because you failed to warn them of your intentions is bad for the environment and bad for traffic flow.