What's to prevent the company form writing off the perk and counting it against the employee's "real" salary?
Different kinds of expenses (benefits and salaries are expenses) have different tax consequences. Things that are on the pay stub may be taxed differently than things that aren't but I don't think that you're going there.
Are you thinking that telling employees "here's how much I'm spending on you that doesn't appear on your pay-stub" is going to have some effect? If so, I suspect that you're about to learn that some folks would rather have that money in their paycheck and that most will tune you out.
Offering benefits allows both companies and employees to agree on a lower cash salary; it's in the company's interest to provide benefits that have a large perceived value for minimum cost, whereas it's in the employee's best interest to only offset his desired salary by no more than the amount that the benefit would cost him were he to purchase it individually.
Complicating matters is the fact that the company wants to provide benefits that provide greater utility to the employee than the cost to the company, but smart employees will take that into account.