The biggest problem with a standard is competing standards.
That being said, having different shapes makes it harder to measure things by eye. Even if all of your cups are the same volume, if they are different shapes that makes it harder to measure things consistently by eye. So I might make the same amount of coffee every day, using my carafe to measure the water and a scoop to measure the grounds, but unless I whip out measuring spoons for the milk I’m relying on a consistent container for that piece.
The biggest problem with a standard is competing standards.
That being said, having different shapes makes it harder to measure things by eye. Even if all of your cups are the same volume, if they are different shapes that makes it harder to measure things consistently by eye. So I might make the same amount of coffee every day, using my carafe to measure the water and a scoop to measure the grounds, but unless I whip out measuring spoons for the milk I’m relying on a consistent container for that piece.