You can deduct only the amount of your medical and dental expenses that is more than 7.5% of your adjusted gross income (Form 1040, line 38). A HCSA reduces your taxable income by the amount you decide to fund it, and you do not have to fund it entirely at the outset of the year (you can make 12 monthly payments totaling the amount you specify for the HCSA for the tax year).
A HCSA and taxable interest bearing savings account do not compare apples to apples.