So, when someone says this, how do we know if they mean it is useful or not?? Because the two translations means two different things in English...
"souvent peu utile" = often of little use
that is an euphemism, meaning the same thing i.e. nearly useless.
A person who says this is focused on the uselessness of the expression, but allows room for doubt about the uselessness being 100% of the time.
The two Correct Solutions given are in contradiction of each other. More importantly, translations such as "rarely useful", which correspond to one of the two, are not being accepted.
your proposal is not wrong, but it was apparently not what Duo was expecting, i.e. rather a word for word translation.