Snap shackles probably provide a more reliable means of releasing from your chicken loop than manual unhooking in an emergency. In extreme conditions such as lofting you may not be strong or fast enough to unhook, it may twist tightly on to your spreader bar hook and finally in the act of unhooking you must power the kite up even more by sheeting in. A mechanical release, as long as it works, may make all the difference.
The downside of mechnical releases such as snap shackles is that they may not work. They may not work because they are rigged incorrectly, that is upside down or the release portion is not in contact with a steel carabiner or metal linkage of proper diameter, they may be corroded or locked closed by sand or finally you may break the release string by pulling too hard. Careful configuration and maintainance is all that may avoid the above problems.
Or, if you are violently or suddenly lofted a short distance to impact as in KSI case #'s 30, 28, 22,12, and others, you may not have time to react and release the kite. In these cases you should neither be hooked in nor shackled in. Ideally, the conditions that lead to lofting like this should be strictly avoided by careful kiteboarding practice. Refer to the Safe Kiteboarding Guidelines for some ideas in this regard at:
http://groups.yahoo.com/group/kitesurf/ ... formation/
So, ideally you never should be hooked in or connected to your kite while upwind of hard objects. If this practice is adhered to, flying overpowered becomes infeasible in that you need to hold the bar. You can fully depower with the triming strap but this may not be sufficient if overpowered enough. So, it all a matter of tradeoffs, how much risk you want to assume and luck if you chose to hook or snap shackle in upwind of hard objects.
This sport is not as easy as it appears, at least to pursue it with a reasonable degree of safety in more windy conditions. It takes adequate training, knowledge, judgment, safety gear and technique to have safe sessions over the long term. If you use a snap shackle, rig it correctly and take good care of it so that you may be able to rely upon it if things go bad.
In case use good kiteboarding techniques and judgment. They will go a lot further in keeping your safe and uninjured than any safety device.