James Kwak gives us his back-of-the-envelope estimate:
The median family household had income of $62,621 in 2008, which means it has a marginal tax rate of 15%. (We’re pretty close to the 25% threshold, so I’ll use 20% in what follows.) So without the exclusion, the typical family plan would cost about $16,000 in pretax dollars, not $13,000; the exclusion gives the median family a discount of 20%. Only about 60% of people get health insurance through an employer plan, so the average discount across the population is only 12%. Given that the price elasticity of health care is almost certainly a lot less than one (if you double the price, demand won’t fall in half), the overconsumption due to the tax exclusion must be less than 12%. Yet our per-capital health care expenditures are more than 60% above those of any other advanced country.
In other words it matters, but not as much as many people claim.
Addendum: Here is Henry Aaron's correction.