Simply put, an interface of the form
```
property(n, cells, result)
```
implies that there is (or might be) some kind of spatial variability in the `property` but does not say what the variability *is*.  Your suggestion, on the other hand, implies that every time we want to calculate the `property` we first have to (correctly!) deduce *all* possible quantities that *could* influence the result, compute the values pertaining to the current location and then pass those into property calculator.  Basically, a single-statement (function call) becomes a nest of branches that's *very* specific to the ECLIPSE fluids at every call site.