Let's take a good look at the Pokemon that get Own Tempo:
Glameow/Purugly: If swagplay forces you to use a generic Normal type with bulk worse than Lucario, barely passable offense (82) and okay speed, something's very wrong here. "Hurrdur i can switch into Swagger" you might say, except, face it, Glameow is doing jack back to Klefki and co.
Lickitung/Lickilicky: Unless you run some kind of full-on support set or a Special attack with 0 investment in attack, Foul Play will still do a good number on you. And, again, if you're forced to run LICKILICKY in OU something is very wrong here.
Lilligant: lol
Slowbro/King: Because beating a potential Swagplay team warrants use of Own Tempo over Rengenerator.
Smeargle: lol
Spinda: lol
Spoink/Grumpig: I don't know, I'd rather give my Special Tank (lol) 2 more resists instead of immunity to something that it should have no buisness staying in on anyway.
Ludicolo: Because switching into Swagger is FAR MORE important than sweeping with Swift Swim or being bulky with Rain Dish (lol)
Just in case you question my inclusion of pre-evos, Numel has been brought up a few times in this thread both seriously and jokingly.
And if people want to bring up Magic Bounce.
0 Atk Klefki Foul Play vs. 252 HP / 252+ Def Espeon: 86-102 (25.7 - 30.5%) -- 1.6% chance to 4HKO after Leftovers recovery
0 Atk Klefki Foul Play vs. 4 HP / 0 Def Espeon: 132-156 (48.5 - 57.3%) -- 41% chance to 2HKO after Leftovers recovery
Calculated vs 252/252+ (lol) Espeon too because people will inevitably bring it up. Putting aside the fact that Espeon is a subpar mon to begin with, most Espeon will still struggle, and what is it doing back to the likes of Thundurus and Klefki, really?
0 Atk Klefki Foul Play vs. 252 HP / 252+ Def Xatu: 82-98 (24.5 - 29.3%) -- possible 5HKO after Leftovers recovery
0 Atk Klefki Foul Play vs. 252 HP / 4 Def Xatu: 130-154 (38.9 - 46.1%) -- guaranteed 3HKO after Leftovers recovery
Xatu can admittedly retaliate with Heat wave, but remember that if you go max defenses it's not really going to do much to skill keys and united nations of skill.
0 Atk Klefki Foul Play vs. 4 HP / 0 Def Mega Absol: 87-103 (31.9 - 37.8%) -- 93% chance to 3HKO
0 Atk Klefki Foul Play vs. 4 HP / 0 Def Mega Absol: 73-86 (26.8 - 31.6%) -- guaranteed 4HKO
First one is vs a physical attacking one and second one is vs a full SpA one. Putting aside the fact that you have to Mega first, you have to rely on generally inaccurate moves (Fire Blast, Play Rough) to hit the swagplay mons and they WILL do a number to you before they go down.