> Instead of a field, you get a field and two methods
And if you wanted a publically read-only/privately mutable value without properties you'd have to write ... a field and one method. Possibly two methods if you wanted some shared validation code everywhere you assign the field.
Like I said, in practice, it's the same. If you have some case where three vs. two members makes a difference you're in pathological case territory.
Though I doubt this makes a ton of real world impact, performance wise. Stripping names would do more to shrink the .Net code.