I don’t think there’s anything particularly idiomatic about “is a,” but that’s a side issue. What’s at issue are the underlying linguistic mechanisms. How do they work? Sure, some communicative tasks may be more common than others, and that is something to take into account. Linguistic mechanisms that are used frequently tend to be more compact than those used less frequently, for obvious reasons. Regardless of frequency, how do they work?
I don’t think there’s anything particularly idiomatic about “is a,” but that’s a side issue. What’s at issue are the underlying linguistic mechanisms. How do they work? Sure, some communicative tasks may be more common than others, and that is something to take into account. Linguistic mechanisms that are used frequently tend to be more compact than those used less frequently, for obvious reasons. Regardless of frequency, how do they work?