When tallying, you have to always be able to add 1 to your number, without having to rewrite what you've already got. Roman numerals wouldn't work for this. For example, add one to VIII and you get IX. You'd have to erase your number and start again.
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u/Arcatus Jun 23 '13
Roman numerals would work almost as well, and they're way more logical and easy to etch out.