While lines of code is not a good way to measure productivity, it is not useless. I’ve used it to uncover situations where people were struggling with work. It’s a heuristic which can be used to draw your attention to areas which could be problematic.
Ink coverage could be used the same way here.
I know what you mean and I agree, but your example of measuring ink usage reminds me of measuring a programmer's productivity by counting the number of lines of code they've written. What would be the equivalent of deleting lines, though?