What a weird thread.

On chain tension, street riders tend to like their chains very tight both because they think it lets them skid faster and because they think they will drop a chain otherwise. On the track, we mostly ride with as much slack as we can get away with. Again, it's personal preference, with some riders preferring more tension, some less. What less tension gets you is a smoother feeling with less jerking of your legs (important for high cadence high power racing) and less wear on the equipment. Frankly I never found that high tension ever improved one's ability to skid or trackstand.

And as for a chain derailing, you can have about an inch of vertical play in the midpoint of the chain and not have a derailing problem. I always test it, and you can, by taking the edge of your track nut wrench and pushing against the side of the chain where it engages the chainring, as you're rotating the cranks slowly forward. If you can't push the chain off with a prybar, do you think it'll come off on its own? No way. Frankly, most people who have a chain come off either messed up the process entirely or had their rear wheel slide forward in the stay ends so all tension was gone.

As for tensioners, yes, they can work, but you don't need them and can learn to adjust your wheels just fine. They cause the spacing in the back to be ever so slightly greater, so putting your rear wheel on is more of a pain. My track bike is lined up on a rail with fifty others at every training session or race night, and nobody has chain tensioners.