In a perfect world you would not have this kind of situation, unfortuneatly we do not install in that world. Often homeowners or " professionals " will replace defective heads with whatever is available on their truck or in the local hardware.
In some instances it makes sense to use a combination of impacts and rotors. Impact heads give the most even water distribution. That means that if you have an area that is too large or has too much traffic to effectively put in pop up heads, but you can't get matched precipitation on rotors then impacts are the next best solution. I am not saying that this is the way to go, only that sometimes it is the best solution to a bad problem.
The harder I work
the luckier I get.