In many cases, it isn't. Free instruments often have serious issues that would be very expensive to repair, and often, the cost of the repairs would exceed the value of the instrument itself. You're better off using that money to purchase a piano in good condition. However, ask a qualified technician to give you an overview of the free instrument's condition and an estimate on any repairs. Only accept it if the instrument is in good shape, is worth a lot (which it probably isn't), or has sentimental value.