With a sharp pair of quality grooming scissors, you can take as much or as little length off your beard as you'd like, and achieve the shape you desire. Scissors are also great for targeting the stubborn split ends that grow horizontally out of your face. They key to success is to be patient and always take a little off at a time.
Another attractive aspect of scissors is you can take as much length of your beard as you'd like without making it appear thinner and less dense. If you are going for length, don't bother with your trimmers, use your scissors to maintain your beard. Snip away any visible split ends without taking length away from the beard.
Many barbers use trimmers to cut the shape they're trying to achieve. They also use them to taper the sideburns, and sometimes the neck and cheek line. Once the shape is complete, they typically go back with scissors to take aways any scrappy hairs. Scissors are also a great tool to maintain a large moustache.