What are the merits, other than being able to ultimately pour the wine from a cleaned out original bottle, of double-decanting relative to those of decanting (defining the later as pouring just once: from bottle to decanter)?
That’s pretty much it. If you’re drinking just one or two wines, and you don’t mind pouring from a decanter, then just a single decant into the decanter makes sense. For the times when you have several wines, and not enough decanters (or not enough space for decanters, or don’t want to worry about confusing the wines), or you’re decanting before taking out to dinner, or to a wine event, then double decanting makes a lot of sense. There are probably some arguments about exposed surface area in the decanter vs. the bottle, but I would argue that once you decant, the wine is seeing about as much air as it’s going to see, either way.
We decant either to expose the wine to air or get it off its sediment, and it goes double (back into the bottle) only if we’re then taking the wine to another venue, usually a restaurant.