There is laying down and there is laying down. I have seem people lay their horse down gently, using a cue. The horse is obviously comfortable doing so and goes down easy. This I have no problem with.
Others almost throw the horse down. It almost looks like the horse is falling, and you often hear a thud. I am personally not comfortable with this.
Either way, a horse laying down is in a very vulnerable position. One that willingly lays down and is comfortable with it has built a bond of trust with the trainer.
From a practical standpoint, having a horse that lays down on cue can be very useful. Especially on trail or endurance rides. In the event of an injury, it could be helpful and beneficial to the horse to lay down until help arrives.
At the Midwest Horse Fair, the driving team demo had a horse get caught up in the rig. As I recall, the horse next to it was cued to lay down, where it stayed until the situation was remedied. Another practical use for laying down, if not endurance related.
Then there is the situation where hostiles are over the ridge and a standing horse will give away your position. You lay him down while you crawl on your belly ot the ridge line and scout the situation. Ok, maybe this one is 100 years past its usefulness. 
Remember, everything we teach a horse is a “trick.” Move forward, back up, lead change, sidepass, lay down - it is all the same to the horse. We give a cue to ask for a specific behavior.
Virtually any cued behavior we teach a horse can have some usefulness in the right situation. Heck, look at dressage. Outside of the ring, these moves are arguable not practical. Still, they are just descendent from medieval calvary manuvers. If you want to teach your horse to attack at predators on trail, there is a dressage move designed for that. I forget what it is called, but it involved a type of rear with striking of the front hooves. This might be safer and more practical to teach than a bucking strike.