Some are defensive some are offensive. Someone like Qualcomm uses them to get a percentage of every device but also uses them to protect themselves against other companies, as they also sell chipsets. Also patents expire. So they may have one but after the time limit they are not really worth much anymore other than a defense. Would limiting the number they hold really help? Think one chipset they had to pull was over some what most people would consider the most tiny of features. Plus limiting would not really put a dent in many of these companies I am sure they would think of some way around it.