I am using the ISY99 to manage Insteon devices and scenes. It sits between OH and the PLM. Uses a different binding.
The ISY is really necessary in a larger Insteon network - devices fail and an easy way to replace them makes managing the network helps.
In OH, I mostly reference scenes rather than Insteon devices directly. That way if (when) a device fails, I just need to use the “replace device” feature on the ISY99 and not change code in OH.