I'm in favor of the US switching to metric (I'm American, but have lived in Europe for the last 7 years), but just trying to give some context on what I believe are some of the niceties of the Imperial system.
A "cup" is a much easier notion to get your head around than "about 50-70 ml".
This opinion can only come from exposure. Whenever I see "one cup" in a recipe I'm 100% baffled. A big cup? A tea cup? One of those larger coffee mugs? Trying to judge what is actually meant here can leads to volumes differing about two to three times in scale.
Give me centiliters, desiliters and liters any day.
I still have no idea what a cup is. And 50-70 ml seems way too little to make sense, so I will just assume you pulled those numbers out of thin air.
It does not matter if you use a big cup, a tea cup or a coffee mug. The meal will still taste good, as the proportions are kept.
While for a 'metrical' recipe you need something that can measure weight and volume, for a 'cup' recipe you just need a cup, no matter what kind of cup.
Fortunately, all ingredients in a recipe are usually given in 'cups'.
Not even remotely true. Especially with "cups" of water and "tablespoons" of chilli the results can be rather interesting ;)