When I'm replacing a rear wheel or just re-tensioning a slack chain, I use my left hand to keep the wheel centered between the chain stays. I use my right hand to dial in the tension. When I've got it right, I can tighten the track nuts without also having to maintain the chain tension. The tensioner does that. I can see that a bike mechanic, who does this operation all the time, might find it simpler not to have to deal with the tensioner. But for those of us who are only working on our own bikes, I think the "third hand" can be very useful. It is for me. And, as I said above, it will help hold the tension longer so you don't have to re-tension so often.

If you need doodads to hold the chain tension while you tighten down the axle nuts, you're probably using too much tension.

Not necessarily true. My wheel creeps forward every now and again but it's just part of riding a bike (like needing to fill your tires every couple of days) and not a difficult thing to correct.

My experience with chain tensioners is that they create more work than they eliminate. I actually found them to be a PAIN in the butt for the tensioning process and an even bigger PAIN for wheel removal and install. The only good thing about them is they do prevent the wheel from sliding forward in the dropouts, but for me the added hassle isn't worth it.

I use my foot to keep the wheel pushed back and chain tensioned, one hand to keep the wheel straight, and the other tighten. Seems pretty easy to me.

This.