I don't think anybody has said that employees would have automatically gotten the other 6% as a salary increase. That money, though, could have been used to grow the business, benefiting both the company and its employees.
Yes, it's been said and implied here and elsewhere that the 12% total that goes to SS would have been compensation to the employee if not for SS.
But if Obamacare goes into effect, we'll see if employers who drop their medical care benefit and pay the fine, or tax in lieu of providing coverage, we'll see if they all give raises to their employees equal to the difference between the tax (10% of payroll?) and what it would have cost to provide medical coverage.