Just the classic problems caused by inconsistency in code - confusion when reading, and disagreements when writing.

For reading: Where should I look in a file for the sentinels? Is this value a sentinel, or is it some other type of variable?

For writing, there will always be disagreements in PRs, so we should nip it in the bud.

You raise a good point about exposing sentinel values. I was previously on the fence about whether sentinels should (in general) be private, or public.