Well, the way the FSA works is that the money is put into your account through pretax deductions from your paycheck. So, if you were in a 25% tax bracket, you would have to earn $3333.33 to take home $2500 (.75 x $3333.33 = $2500). Thus, you are saving $833.33 by putting the money into the FSA, provided you use all of it by the end of your company's FY.
I'm not sure I agree with your math, you are having $2,500 removed form your paycheck pre tax so at the 25% tax bracket you are only saving $625 in taxes. This assumes you max out the FSA, now would you really spend $2,500 a year in medical expenses or are you spending $2,500 because it is in you FSA. And of course the tax savings is decreased when you have less taken out of your paycheck and put into the FSA.
My issue the the FSA reimbursement wasn't the OTC / debit card but was with other medical co payments and such. My company switched providers and the new one made that whole process a lot harder. They did not allow us to use the debit card for anything other then prescription / otc uses.
My point is that the IRS has made these way less attractive by removing the OTC use.