I think that’s why the threshold is not set to only show 100% translated pages. That way visitors can see that a translation exists and that it’s incomplete, and hopefully will then contribute the missing pieces.
As a reader, I personally prefer the inconvenience of seeing such pieces of untranslated content on the live site and having the ability to quickly fix them, than not seeing the translation at all.
As a contributor, I also like this approach because making the gaps very visible reduces the chance that large blocks of untranslated text would accumulate, which makes the translation process less appealing. By failing fast and often, we ensure that we also fail small, and are able to correct quickly.