One advantage to consider is that there is no time limit on when you spend the money. As you age, your health care costs will rise. How much interest are you really making on $5000 in today's environment? $50? $100?

Meanwhile, you save your marginal tax rate on any dollars contributed towards future medical expenses. That's a pretty good return on your money. $5000 at 28% marginal rate knocks $1400 off your tax bill.