I focus on light roasts because the light roast brings out the specific flavors and aromas of the individual coffee varietal, whereas darker roasts tend to make all coffees taste similar; charred, smokey, and 'roasty.'
There's also a very specific chemilcal reaction that occurs about 1/3rd of the way into the roasting process and that reaction is called the Maillard reaction (pronounced "Mayard"). During the Maillard reaction, sugars, amino acids, and water are converted into other compounds and are therefore not available later in the roast. Because I want a sweeter coffee at the end of the roast, I want to leave just the right amount of sugar and water for later roast phases. Therefore I normally scoot through the Maillard reaction as quickly as I can.
The final phase of the roast, called the roast development phase, carmelizes sugars and if you've ever tasted carmelized sugar — like what's on the top of a creme brulee — you'll know it is bitter-sweet and carbony. That same flavor and aroma is developed in the final phase of the roast and for that reason, I don't want to spend too much time in the roast development phase either.
There's nothing wrong with darker roasts. In fact espresso beans are normally a darker roast but for a sweet and complex cup of brewed coffee, nothing compares to a good, light roast.