I have mixed feelings about that video. On one hand, there is no doubt that a 5-point harness offers better protection than a 3-point seatbelt; that's why race car drivers use 5-point harnesses.
On the other hand, there is no way a properly used seatbelt should have failed in a crash like the one described in that video. Either the belt was improperly fastened (user error), or the mechanism failed, and the vehicle manufacturer should issue a recall to replace the mechanism in all their other vehicles to prevent a similar malfunction in the future. Even a 5-point harness seat must be properly secured to the vehicle to work. If the seatbelt isn't fastened or the buckle mechanism is fault, the harnessed seat will also be thrown from the vehicle, and ejection significantly increases the risk of death in a motor vehicle crash.
The top tether is *not* a substitute for proper safety seat installation or seatbelt use. The top tether serves to decrease head excursion, reducing the risk of injury, especially head injury. It is interesting to note that the shoulder belt portion of a seatbelt used to install a front facing 5-point seat will also help to reduce head excursion (when compared to a lap belt only installation). "Best practice" is to tether the seat if a tether anchor is available, because it improves the performance of the seat.
Many vehicles that don't have anchors installed can be retrofitted with *top* tether anchors. The cost varies greatly, from free to several hundred dollars. A tech with a LATCH manual can tell you if your vehicle can be retrofitted and give you the part numbers if your vehicle manual is unclear. Frustratingly, service centers often don't know anything about top tethers, so it is usually best to educate yourself before taking your vehicle in. (Please note that only a few vehicle models can be retrofitted with *lower* "LATCH" anchors. See
http://www.car-safety.org/latch.html for more detailed information.)
Even an untethered seat provides excellent crash protection when properly installed. Proper use is the key, whether you have a rear facing seat, a front facing seat, a booster or a plain ol' seatbelt. You need to be sure that whatever restraint system you use is appropriate for your child's size and current stage of development. For example, a large 3 year old may fit well in a booster seat but be unable to sit still for the entire trip, making a 5 point harness a better choice. A large 2 year old may be too heavy for a rear facing seat and need to sit front facing, while a small 4 year old would still fit and be safer rear facing. As always, the *best* restraint is one which:
- Fits the child.
- Fits the vehicle.
- Fits the budget.
- Will be used properly every single trip.