Not just commercial photographer, timing is an important factor for all kinds of photographers dealing in different branches of photography. More the conditions get volatile; more is the importance of timing. It is one of the first things that a photographer ought to develop and get efficient at. Good sense of timing comes with lots of practice and experience. Timing is particularly required in outdoor shoots, when weather conditions are not stable, and with unprofessional subjects like babies and animals.
This is yet another tough job, but very critical. This holds true more for the commercial photographer than for photographers working for any other offshoot of photography. Why? Because, in commercial photography, one shoots for advertising purposes, where there is a target audience and they have to be convinced with action and expressions. Therefore, it is imperative on the commercial photographer to bring out the right expressions in his subject.
There are two kinds of models that a commercial photographer usually works with. One is the professional model (that can be both kids as well as adults). And the second is the not so professional ones or those who are too incompetent to understand the whole idea of professionalism, like animals and babies. In the first case, the commercial photographer does not really face that much of a difficulty as the professional models usually bring forth the required emotions and expressions with little effort. However, it is in the second case where a commercial photographer needs lots of good sense of timing and the expertise to help the subject emote in the desired way.