Is belief in eternalism completely useless?

The false view of eternalism is not skillful and not good, however, a belief in a permanent self can be repurposed for good.

A view of a continuously existing self can be used to generate the path to liberation, using the contemplation below. Therefore, it's skillful means. It's typically targeted at those with self view.

(1) “This noble disciple reflects thus: ‘I am not the only one who is subject to old age, not exempt from old age. All beings that come and go, that pass away and undergo rebirth, are subject to old age; none are exempt from old age.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.

(2) “This noble disciple reflects thus: ‘I am not the only one who is subject to illness, not exempt from illness. All beings that come and go, that pass away and undergo rebirth, are subject to illness; none are exempt from illness.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.

(3) “This noble disciple reflects thus: ‘I am not the only one who is subject to death, not exempt from death. All beings that come and go, that pass away and undergo rebirth, are subject to death; none are exempt from death.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.

(4) “This noble disciple reflects thus: ‘I am not the only one who must be parted and separated from everyone and everything dear and agreeable. All beings that come and go, that pass away and undergo rebirth, must be parted and separated from everyone and everything dear and agreeable.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.

(5) “This noble disciple reflects thus: ‘I am not the only one who is the owner of one’s kamma, the heir of one’s kamma; who has kamma as one’s origin, kamma as one’s relative, kamma as one’s resort; who will be the heir of whatever kamma, good or bad, that one does. All beings that come and go, that pass away and undergo rebirth, are owners of their kamma, heirs of their kamma; all have kamma as their origin, kamma as their relative, kamma as their resort; all will be heirs of whatever kamma, good or bad, that they do.’ As he often reflects on this theme, the path is generated. He pursues this path, develops it, and cultivates it. As he does so, the fetters are entirely abandoned and the underlying tendencies are uprooted.

AN 5.57

There is definitely a sutta that says annihilationism is the most beneficial of outsider wrong views because at least annihilationists are expected to have developed some dispassion.

Where as I have never read a sutta praising eternalism.

"In his effort to master kamma in such a way as to bring kamma to an end, the Buddha discovered that he had to abandon the contexts of personal narrative and cosmology in which the issue of kamma first presented itself. Both these forms of understanding deal in categories of being and non-being, self and others, but the Buddha found that it was impossible to bring kamma to an end if one thought in such terms. For example, narrative and cosmological modes of thinking would lead one to ask whether the agent who performed an act of kamma was the same as the person experiencing the result, someone else, both, or neither. If one answered that it was the same person, then the person experiencing the result would have to identify not only with the actor, but also with the mode of action, and thus would not be able to gain release from it. If one answered that it was another person, both oneself and another, or neither, then the person experiencing the result would see no need to heighten the skill or understanding of his/her own kamma in the present, for the experience of pleasure and pain was not his or her own full responsibility. In either case, the development of the fourth type of kamma would be aborted [SN12:67, Udana6.6]."

~ Thanissaro Bhikkhu "Kamma & the Ending of Kamma" https://www.dhammatalks.org/books/Wings/Section0009.html

Is belief in eternalism completely useless?

Yes, it's completely useless. It will only keep the mind in delusion and away from clarity, wakefulness and the realization of wisdom.