I do conversions all the time: I cook. I look up recipes. And US/Canadian recipes are written with deeply moronic idea that quantities of matter are easily measured by volume.
So I have this table of how much grams is a bloody cup of strawberries (and I don't know the kind of mental confusion which may lead anyone to think this makes the remotest amount of sense). Because if you want to be somewhat precise, there is only one single appropriate tool in a kitchen, and it is the scale.
Likewise, it is easier to mix liquids in ml (grams if it is water) and solids in precise amounts by weighing. And most importantly, this allows you to scale the recipes easily by the amount of the main ingredient.
As for the Fahrenheit thing, all I can say is that it is a truly moronic scale. From the freezing point of saturated brine to the body temperature of a human with a slight fever. Obviously this makes sense -- not. Feet, inches, miles, pounds, you know what? its some arbitrary choice. Fahrenheit is just dumb.
But whatever, these conversations usually boil done to this bizarre fact: Europeans like decimal notation, and Americans like fractions. And for sure, if you like fractions, you probably think conversions are not too useful. I can also tell you are American by the fact that you think "KMPH" is not disturbing as a notation: even in science, I find that Americans can think of acronyms as single entities, e.g they read ABC/DEF as "ABC"/"DEF", whereas Europeans will introduce symbols such as A_{foo}/B_{bar}.
As far as I can tell, this is the deep root of why notations/units are such. But the fact remains that measuring quantities of matter in volume is wrong :)