This would be a great tool, gr5 I agree that is a good way to measure, especially if its a square/box but i would like to measure holes inside the project, and there's no easy way to do that.
BUT! i have found a temp solution, you can create square box by pressing E (where you choose if you want it to be "print as model or print as support") it creates it 10mm x 10mm x 10mm but you can resize it to what you want the hole to be, and then move it in the hole to measure it.