Most commercial producers use cross bred stock. The reason this is the case is duet to an occurrence known as "heterosis" or hybrid vigour. As a result of cross breeding from two different purebreds such as a Rhode Island Red crossed with a White Leghorn, the offspring are often genetically superior and as a result of hybrid vigour a number of factors such as disease resistance, egg laying rate, behaviour, feed to meat conversion and general health can be seen to be markedly better. Most people tend to keep purebreds for ornamental purposes and as show birds rather than for their eggs and meat.
The other bonus for hybridising pure breds is that you can choose the positives of each breed that can then be passed to the offspring, which gives a farmer greater control in finding an appropriate breed for their circumstances, be it disease resistance, certain egg size, output or egg aesthetics.