If I recall correctly, I believe that swellow is the fastest non-uber pokemon who has access to sunny day (not sure about rain dance though) this generation (deoxys also gets it, but he'll probably be banned soon anyways). Just something to think about. Everyone keeps saying how ninetales outclasses him in terms of this, but you also have to remember that EQ, rock, and water type moves are extremely common in OU, so it will be harder to safely switch him in. Swellow, on the other hand, is only weak to one of these common move types. The other move types he is weak against are slightly less common in OU that the types that ninetails is weak to.
You're forgetting how frail Swellow is. He can't take hits much better than Ninetales anyways, and even if Ninetales gets knocked out as he is brought in, Drought will still get set up. Swellow running weather completely ruins his niche. Also, Swellow itself has ABSOLUTELY NO USE with Sunny Day or Rain Dance, so he should be ignored as an option when fast users like Electrode and Jumpluff can set it up AND benefit from the weather effects.
Yes, Swellow can learn Rain Dance and Sunny Day. Yes, Swellow is fast enough to set up. But Swellow is dead weight on a weather team because of its frailty and not being able to use the weather it's setting up.
Swellow needs to stick with Guts Facading. That is the only thing he can do successfully without being outclassed.