Anyone who has maintained Makefiles will have a strong opinion on semantic whitespace.

Maybe not opinion, but reaction. Generally involving visceral fear or anger.

It's not that it's always bad, every hour of every day. It's that one time, at 1am, when you shouldn't be working but you are, something doesn't behave properly, and it turns out Bob the Intern re-indented a block of code to use tabs instead of spaces[1], but you couldn't see it because your editor is configured to use four spaces per tab and your coding standards dictate four spaces per level of scope.

[1] Why are you using spaces anyway, you complete and utter savage?