I see it everyday, someone will change lanes and only use their turning signal during the lane change. Often there will only be one or two light blinks while the car is straddling the lane divider. This is basically pointless and it leads me to believe that many drivers see the turning signal as a polite formality that they owe the other nearby cars.
But the turning signal is not a formality, it is a tool that mainly lets you indicate that you are going to turn in the very near future. The turning signal is helpful during the turn, but it is much more important to use it before you turn. You want to give other drivers a heads up before you turn, not while you're turning.
A lot of the time I will put my blinker on before I even look over my shoulder, then I will look around, then I will start to change lanes, and I often turn my blinker off about halfway through the lane change (because by that point its job is done).