Who said he has to be at +6 to tear holes?If it carried taunt, I could see it, since it isn't exactly common anymore. But, as soon as it starts NP'ing someone in their right mind would switch into their attacker, and stop it. You literally have to be at minimum competent, to stop Thundy before he fires off a +6 TBolt.
+2 252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 252+ SpD Sylveon: 222-263 (56.3 - 66.7%) -- guaranteed 2HKO after Leftovers recovery
Even their best fully invested special wall is 2HKO'd after a NP (and they sometimes go physically defensive), so they can't just "switch in and stop you", they're getting smashed by that. All the other things on their team are pretty much 1HKO'd by +2 Thundy, and losing Sylveon can be disastrous for BP if their Espeon doesn't have a secondary attacking move (which it often doesn't because it likes the utility that CM and Substitute offer), since then Bisharp completely walls it, and Zapdos/Vaporeon aren't enough to trouble offense with their relatively feeble attacks.