The goal is to match the speaker to the task it was designed for and make sure that you never apply frequencies inappropriately to speakers that cannot handle it. A first order crossover is often what you will find in PA systems. The reasons for this are many. There are major differences between the actual speakers found in a PA system versus those used in a Studio monitor or home speaker system.
PA speakers tend to be very high output.
You can use second order crossovers in PA systems, many of the integrated multi-speaker cabinets available today have them. Studio monitors are made for a specific environment, and are focused on getting the most accurate representation of sound in a small stable environment. They must also be able to handle large transients and often substantial power. This is not to say that some PA speaker systems are not highly accurate at reproducing sound, however, it is unlikely that you will find many pro-studios mixing down sound on their outdoor PA systems. The end functions are different, and you should always keep this in mind when designing speaker systems.