Hoc mihi usui est
This is useful to me.
Noli hanc rem mihi vitio vertere.
Don't impute this as a fault of mine.
Hoc magno eis detrimento erit.
This will hurt them greatly.
This predicative dative is frequently accompanied by another dative of the
person concerned.
The idiom covers the ground of the English expressions "serves as ..",
"proves ..", e.g. It proved a hindrance to him; it was to his discredit; it
served him no good.