If a function doesn't modify a passed data frame, R doesn't actually copy it. It's formally pass-by-value, but the implementation uses a copy-on-write approach, so no copy is made when the function only reads the parameter's values. That at least covers the common case of passing a bunch of data to a function that builds a statistical model.

Of course that doesn't help in the case where you do want the called function to modify the data, but in-place rather than by making a copy.