Swift swimmers are fucking amazing, but the only reason they aren't that good is because the only way you're going to be sure they'll work is if you run Rain, but sadly, you can't! Rain Dance is shit because all the swift swimmers need their coverage moves, and it's usually stupid to run rain dance teams in this meta anyway.
Swift swimmers do demolish rain however. Look at Kabutops - LO Waterfall kills Tornadus, Waterfall + Low Kick kills ferrothorn, Stone Edge 2HKOes some Tentacruel... it basically 2HKOes the meta (including Keldeo, which falls to LO Waterfall iirc!)
They're not even that bad out of rain - Kabutops kills Mamoswine with Aqua Jet and checks Heatran pretty well, for example, but in the end they're going to be subpar out of rain, and that can hurt, especially against Deo-D + 5 sweeper teams where you need all the Pokemon you can get.