"I do not shine."
How is it not idiomatic? The literal translation doesn't really make sense, you have to learn it's meaning separately from the words (save for the fact that there is a similar idiom in English - to take a shine to something.)
Even if it is used heavily, an idiom is an idiom. Most idioms are used a lot, in fact.
Dia duit isn't an idiom, it is simply a different phrase altogether - one that is straightforward. A thing doesn't literally shine with you, and saying that makes no sense without learning that is means to like/enjoy something. With dia duit, however, you are saying exactly what is meant: that I wish god be with you, to protect/guide/whatever.
Most people don't attach that meaning to it anymore, it seems, just as with goodbye. That doesn't make it an idiom.