When he said "the processor heats up than cools down fast, its hard to catch it"
I'm sure whats happening is the cpu is heating up but the heatsink takes a second to
take the heat transfer into it. (its not like its at 150 degrees than your dumping ice on it)
In theory, yes. Any object, especially a cpu heating up and cooling down is bad for it.
But i dont think there is a way to avoid the process of it heating up and cooling
down regardless if it is doing it fast or not.
Overall having the heatsink is better, because it disapates the heat easier instead
keeping the heat and causing more ware on the cpu/phone. (cooler/lower degree phone has a longer lifespan)