I think a better (but still simplistic) example is a data type for "day of month". How do you write a validator for that unless it also has access to "month of year" and "year" (just in case we are talking about February)? So why make getting access to those other items difficult?

How do I allow $price->Currency = 'yen'; to know that $price->Amount needs to change as a result?

Now blow that up to cases that aren't so simplistic.