If you tow heavy or have a sloppy stock torque converter, it puts a lot of stress on the input shaft when the converter locks up in overdrive. The input shaft is splined into the drum and the shock load can strip the splines and let the input shaft turn which usually breaks the drum.
If you have less than 300 HP, a new stock input shaft and drum will be just fine. If you tow heavy and/or plan to increase horsepower, might want to spend the extra money on a billet input shaft/drum assembly. A new billet one costs anywhere from $650 to $850 depending on shaft make and where you buy it.