If the batteries are good for 200,000 miles and you get one at 75,000, for half the price of new, and you pay cash (financing on used is typically terrible interest rates), then this is an economical purchase. If they require serious repairs other than the battery then maybe it's not economical.
There's some small advantage to buying used, even if it's not the best deal. It's a fixed amount of money for a fixed amount of time (life left). If you hate the car and want to change it, or it gets totaled and insurance won't pay for it, then your financial hit is less than if you had paid full price. Your next car can still be new, or used if you find a better deal. Finally, if you have the means, there's no shame in wasting some money on something you really want.
However, being used, and being a used rental car, are two different things. Buying it from a fleet, at least it would be driven somewhat sanely. But there's no crazier driver than a rental driver.