What does it mean when Hadith scholars such as Ibn Adi say that a Hadith narrator's hadith may be written despite his weakness? Does it mean that the narrator is not very weak and that his hadith may be used as support?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Some scholars may say about a narrator that his hadeeth may be written but not used as authoritative evidence. They mean that his hadeeth may be used as a shaahid (i.e. a hadeeth with a closely similar meaning reported from a different Companion through a different chain of narrators) to other narrations, so if it is reported through another chain of narration, it is corroborated by it.

Ibn Taymiyyah wrote, "Abu Haatim said about Muhammad ibn Talhah At-Taymi, 'He is classified as a truthful reporter; his hadeeth is written but not used as authoritative evidence.' This means that his hadeeth is valid for I'itibaar (i.e. carrying out a comprehensive study of the hadeeth in order to gather its various chains recorded in the books of Hadeeth so that one may be able to pass a correct judgement on it) and to be used as a shaahid to other narrations. If it is corroborated by another like it, it is permissible to use it as authoritative evidence, but it cannot be used as authoritative evidence on its own."

Allah knows best.