The advantage is the strap prevents the shock from extending too far and binding axles. Over extension of the shock is likely caused by the shock being used in an application it's not intended for or the shock being designed without any limitation on full droop.
The disadvantage is shocks that require a limit strap also tend to require a stack up of washers to fit in the shock mount because they are a generic shock with a poor overall fit for the specific machine. Additionally, the shock mounting bolts are typically relied on for the limit strap mount. A limited strap is intended to absorb the impact load of the suspension as it reaches the limit of travel and in my opinion a bolt in single shear should never be subjected to shock or impact loading. I'd be fairly concerned that consistent impacts during full droop events could bend or break the bolts.
There are plenty of similarly priced shocks that are the right length or offer internal jounce bumpers. With those other options available, many people wouldn't consider the "needs a limit strap" type options. In the end you can have all the shock settings in the world and your ride quality is only as good as how they are set right now.