In my opinion, it makes complete sense. The good thing about a sentence, even if it has no translations, is that it provides context, and sometimes that's all a student needs in order to learn a new word or phrase.

Also, it's possible to translate sentences with wordplay in a way that makes it possible to understand the original meaning, even if some or all of the wordplay is not preserved in the translation.

On addition to Alan's message, I think you cannot be sure that a sentence really cannot be translated. It might be so in the languages you know but some language may have the same trick for the particular sentence.

Other than that, Tatoeba is not a translation platform. From the about page, for example, Tatoeba is a "big database of sentences AND translation", "a tool providing examples of how words are used in context".

So I believe the sentences you describe serve a very useful purpose if they have value in themselves. To be honest, I'd like everybody to remember that not everybody uses Tatoeba for second language study and stop being biased by this erroneous idea.

Erilaiset sanaleikit, idiomaattiset ilmaukset, vitsit ja muut erikoisuudet ovat juurikin hyvää sisältöä Tatoebaan. Niitä voi olla vaikea löytää muualta ja ne ovat osa kielen rikkautta, joka voi muuttua nopeasti tai olla hyvin aluekohtaista.

I saw this recent post:
https://tatoeba.org/eng/wall/sh...#message_34175
and was reminded about this thread here.

First, I think every sentence can be translated. Not every sentence can be easily translated into every language, but everything is translatable in some way or another. Translation is not about re-creating with exact precision a sentence from a certain language into another language. It's about finding the closest match in the other language. There is always going to be a closest match.
Also, unless you can speak every language of the world, I don't think you can claim that there are sentences that cannot be translated. Maybe there will be a sentence that you cannot easily translate into the languages that you know, but who says that there isn't another language out there in which the sentence can be easily translated?

Second, translations are not the only information that we attach to sentences. You have comments and tags as well. Even if a sentence cannot be translated, there can still be valuable information someone can discover about the sentence if there is a comment or a tag. Sentences that are hard or almost impossible to translate probably carry very interesting features of a language. It would be shame to exclude them.