The primary reason for isolators is to prevent signals from being coupled down the antenna and back into the power amplifier and causing intermodulation products. This comes out in a combiner spec as Transmitter to Antenna isolation. The isolator is designed to circulate reflected or coupled power away from the amplifier source and to a dummy load to be dissipated as heat. The isolator can and does dissipate reflected power, and it is designed to do this, but it is not primarily designed to be a heat management device. I think competition has gotten us all into this little incongruity.
Common practice is to put dummy loads on isolators equal to one-half of input power.
Its not often that we have the opportunity to have a customer get far enough into the technicalities to explain this. An impedance match has to be maintained between all system elements. In cavity combining, there are critical cable lengths between system elements. For example, between two cavities we will try and achieve a 50 ohm impedance match with a phase difference of 90 degrees. In other words, a quarter wave cable.