The short answer is when you understand what it is you are dealing with.
What many do is resort to it as some sort of de facto first action, which can have really dire consequences. If anything the de facto first action, after doing water changes, ought to be lowering the temperature.
I've written it a few times now, maybe one day folk will take heed ... if you elevate temperatures you increase the fishes demand on O2, at the same time you reduce the O2 carrying capacity of the water. The fish will produce more waste just through respiration, i.e. ammonia, which will lower water quality, which in and of itself will place a demand on O2. Add in that most pathogens will multiply faster, this will cause the fish far more stress, which will make the fishes respiratory system work harder, which means it uses more O2 ...
Now lets add some meds ... guess what? many meds will also place a demand on the available O2 present, further reducing that available for the fish, which will stress them even more and cause them to use even more O2, which they don't have by now ...
I'm sure you get the picture ...
The pathogens themselves may well overwhelm the fish before the fishes immune system has spooled up, it is also worth pointing out that the immune system won't necessarily stay spooled up! High heat also stresses the fish. I'd question the value of any immune boost against the risks, particularly when most won't know what they are dealing with.
The commonly stated belief that high heat will kill spiro/hex is not unfounded, but it is misunderstood. 36C will do diddly squat to spiro/hex, it needs to get north of 38C and be maintained at that rate long enough for the fish to equalise and therefore affect the pathogen. I've tried treating spiro/hex with this method and it would be safe to say I've had mixed success. If the fish are in anyway compromised (other than with spiro/hex) they can quickly die at very high temps.
Also at high heat levels you have to witness the speed of decay if a discus dies in the tank. You can quickly end up with a total nightmare.