This. The benefit to the Unsullied is that you don’t end up like Stannis.
Well, they do train day and night from the time they are toddlers so i imagine they ARE badass warriors. They are just not big and tough.
If I recall, there was an English king or warlord or some such who lost an eye to boiling lead during a battle.
In reality, they heated up/boiled whatever was handy. Boiling water, sure, but grease and fat from animals was also readily available and probably the source of “boiling oil” rather than petroleum as a source.
In a siege, water, at least the drinkable kind, could become scarce, so pouring good water onto the enemy might not have made as much sense as pouring, say, the contents of chamber pots onto their heads. Or whatever else noxious or dangerous you might have handy.
Wikipedia notes that a) oil was used but often prohibitively expensive, and b) it was merely very hot as boiling is a misnomer. Would oil also be better than water because it remains hot and doesn’t “wash away”?
Though it’s a tiny snapshot rather than an epic battle, they did do it more or less right in the first episode or two of the series Rome. The Roman troops stay in a tight formation, changing ranks in an orderly fashion based on the barked orders of the centurion. When the resident badass breaks ranks to start slaughtering enemy soldiers via his badassery he gets called back and eventually tosssed in the stockade for insubordination.
The previous HBO series Rome (to which the TV Game of Thrones is enormously indebted) did have some good scenes of legions fighting in formation. (The consequence of breaking formation is actually a plot point at the very beginning.)
Damn you Tamerlane. 
Great minds, etc. etc. :D.