So someone should be not enough educated to use a weak passphrase, but has the competency to recalculate a checksum?

I repeat: it's an advanced feature, and like all advanced features it requires a precise application in relation to goals and circumstances.

For sure, every UI that exposes the passphrase possibility must do it carefully, use the correct terns ("25th word" is just wrong) and point the user to a good informative resource.