If back-stabbing is easier, faster and allowed by the National Electrical Code, why do electricians opt for the more difficult, time consuming job of bending the tip of each wire into a loop and then tightly screwing the wire directly to the device?
It's simple. Back-stabbed wires are more likely to come loose, creating a poor connection, than wires anchored firmly around screw terminals. At a minimum, loose wires can cause a receptacle or switch to stop working. At worst, as shown in the above pictures we took at a recent service call, they can start a fire.
Inside each device is a small, integral spring clamp that the device depends upon to maintain contact with the conductor. When wires repeatedly heat up and cool down, as occurs with a loose or poor connection, eventually the spring holding the wire will experience metal fatigue. The device may either stop working or in a worst case scenario, overheat to the point of catching fire.
A homeowner may notice a loose connection when they have to "jiggle" a plug in order to get the device to work. But often the homeowner has no idea they have a potential problem, even on a back-stabbed device, until a circuit becomes overloaded or near overloaded (such as when a 1500 watt space heater and a 1500 watt hair dryer are plugged into the same circuit).
Devices, especially wall outlets, often experience a lot of "hard usage". For example, think of all the times you have seen someone "rip" a power cord from the wall rather than walking up and unplugging it.
For these reasons, best practices in the electrical industry include securing all wires under the terminal screws rather than using the back-stab method.