I'd rather not have Brave Bird, since I'd rather not OHKO myself, with recoil and Flame Orb. So Pluck is the next best. Swellow isn't the type of Pokemon who can OHKO the foe, and as such I'd rather not really damage with Brave Bird but have Swellow KOd. And just because it has high speed doesn't mean it can't be outsped. Priority, higher base speed, Baton Passing speed boosts,speed boosts in general, Choice Scarf, all these things make it so that Swellow can be outsped a fair amount actually. So that is where priority comes in. And its not like it has other moves. Steel Wing is possible, but I'd rather not have Steel type coverage, seeing as Rock types usually have high defense and can take multiple Steel Wings and Ice types have poor defense and should be taken down by Facade.