The wine is going to have air contact regardless. The decanting funnel splays the wine against the side of the decanter rather than dumping it into the center and creating a lot of bubbles.
But if it's for aeration because you think it will help, there's no reason not to simply dump the wine into the decanter. If you're decanting for sediment, it doesn't matter - you decant until you start seeing sediment. And if you're actually worried about losing the nuances of a fragile, old wine, don't decant at all.