I admit it, I don't use my decanters all that often. But when I do, I wonder why I don't do it more often.
The science of decanting isn't all that clear-cut. While many experienced tasters, whose opinions I respect, are convinced of the benefits of decanting, I can't come up with many good scientific explanations for why it might enhance the character of a wine.
The received wisdom in the trade is that decanting 'opens up' the aromas of younger, more structured wines that are otherwise a little closed; that it changes the structure of tannic wines, making them more rounded.
And, from my experience, I'd agree. The problem for my science is that decanting often has an effect almost instantly, yet oxidation reactions in wine take much longer to have their effect. Decanting the wine would saturate it in oxygen, but this would take quite a while to work on wine components.
Perhaps some of the benefits of decanting are the effects it has on us, the tasters. After all, wine tasting is an interaction between the wine and the taster, with both contributing. If the act of decanting changes our expectation of the wine, then it may change the taste.
The wine in my decanter at the moment is Poliziano's Vino Nobile di Montepulciano 2005, which I bought in Tuscany in June. It's a good wine: there's some stylish oak complenting the grippy Sangiovese fruit, and while it's pretty tannic, it's quite sophisticated, too. If anything, overnight in the decanter has improved the wine a bit (and 24 h will have been long enough for some oxidative changes, but in a young wine like this the oxygen will probably have been absorbed by the phenolic compounds).