Preening has a few purposes; cleaning is most obvious, and beside that are straightening out disarranged feathers and ensuring that secreted oils are evenly spread for waterproofing.
For the swift in cruising flight, none of these requirements is likely to apply; the slipstream itself will help a great deal to keep the feathers in aerodynamic conformation, and dirt, parasites, and bodies of water are rarely found at altitude. (Rain is a plausible concern, and I'd be curious to know how swifts cope. Given their small size and altitude of flight, I wouldn't be surprised to learn they rarely notice any but the heaviest of storms.)
For the swift in cruising flight, none of these requirements is likely to apply; the slipstream itself will help a great deal to keep the feathers in aerodynamic conformation, and dirt, parasites, and bodies of water are rarely found at altitude. (Rain is a plausible concern, and I'd be curious to know how swifts cope. Given their small size and altitude of flight, I wouldn't be surprised to learn they rarely notice any but the heaviest of storms.)