re: Letting wine breathe... real or placebo?

A decanter opens up a lot at the bottom. This allows more of the wine to be exposed to air. I would say it's a combination of the pour and the exposure to air.

I have a decanter that has a "top" to it. You pour the wine into it and it is swirled through tubes with holes in them that mixes air into the wine. The wine comes out of the holes and down into the decanter.

I also have an aerator type device. You pour wine into it while holding it over a glass. It moves around and mixes with air as it pours into the glass. I have found it to make a difference that makes it worth using.