Well you don't switch weather starters in on him. You have PLENTY of chances to switch them in when Keldeo is forced out. I might as well get this impending rant out of the way now. (Side note: It's a damn shame OU hasn't caught into Calm Mind Ninetales like DW has. It really does a number on Rain.)
Instead of just a list of calcs, as someone who knows all the Musketeers inside and out and plays both OU & DW OU, here are my actual thoughts on Keldeo, specifically the over-hyped Specs Keldeo:
After doing the damage calcs for Keldeo, my opinion about Specs Keldeo changed a bit. Specs Keldeo is pretty scary...in Rain. Decent and entirely manageable outside of Rain but most threatening in Rain.
BUT
I'm well aware that Keldeo will be spending most of it's time in the Rain and for the rest of this post, I'm considering him to be in Rain, so don't think I'm taking him lightly because of that.
With that said, comparing Keldeo to Garchomp, Excadrill, or Thundurus as a few of you have is a piss-poor, ill-informed comparison. Keldeo is nowhere near those 3. Specs Keldeo anyway, but CM has it's flaws as well. I'll put it this way: There's a lot more Pokemon that can stop Keldeo than the amount that can stop +2 Garchomp, Excadrill, or Thundurus. I would say "open your eyes", but I'm going to point everything out regardless.
I'm taking Keldeo lightly (compared to most of you) because of one reason: he's easy to revenge. For all those fast Psychic-, Electric-, or Flying-types like Espeon, Lati@s, Starmie, Alakazam, Jolteon, & Tornadus, Keldeo is nothing more than a free Sub or a Calm Mind, and Espeon & Jolteon can Baton Pass that Sub if they choose to while Alakazam & Tornadus can sweep with theirs. 108 Speed one of Keldeo's biggest downfalls. At lot of Keldeo checks are right above his Speed Tier and a good number of them can abuse Rain right back at Keldeo.
To add on to that, there's a plethora of Scarf Pokemon that out-speed and take Keldeo for a ride; Rotow-W, Abomasnow, Magnezone, and Salamence for examples. If you think outside the box a bit, Pokemon like Scarf Jirachi, Scarf Metagross, or Scarf Victini, Pokemon Keldeo knows he can OHKO, can run Zen Headbutt or Fusion Bolt and revenge it while he thinks he's safe. Having 4 weaknesses that nearly any Pokemon can run and no immunities makes Keldeo a lot easier to stop than Garchomp, with his Dragon & Ice weaknesses (YacheChomp make's Ice mute) and Electric immunity, and Thundurus, with his only weaknesses to Rock & Ice & an immunity to Ground. Keldeo is not anywhere near that hard to take down in comparison to them.
But ironically, Specs Keldeo has trouble with Rain teams. Ferrothorn & Politoed, Keldeo's best chances to start doing damage against, more often than not, carry Protect, meaning Keldeo's forced to giveaway his attack against Pokemon he has a massive advantage against. After the Protect, it's just a matter of switching to a Pokemon that resists Secret Sword or absorbs Water. Not to mention that Protect eats up one of Hydro Pump's precious 8 PP.
Keldeo also has trouble against Hyper Offense, Keldeo's "amazing defensive typing" (Please.) leaves him vulnerable to Spikes as well as the STAB of Hyper Offense's hard-hitters. Keldeo needs to come in safely, so when powerful Dragon-, Fighting-, or Ground-STAB is constantly being thrown around, Keldeo can't just switch in as he feels. Hell, his typing doesn't even let him switch in on Stall since he's open to all status aliments.
But the biggest flaw Keldeo has is that it only 2HKOs resistant Pokemon and it's Choiced. Any Pokemon that resists a Choice-locked Hydro Pump or Secret Sword with reasonable bulk can revenge or status (Paralysis is best) Keldeo at the cost of some damage even if they're far slower than Keldeo. Specs Keldeo should never "sweep". He KOs one Pokemon and then runs away. (Or stays in like an idiot) Sound familiar?
Keldeo has to not only switch in safely on a Pokemon that he can wall or OHKO safely, but Keldeo has to make sure that they don't have Protect or a Pokemon that can switch into whatever move you lock yourself into. That's a TALL order for a Pokemon with absolutely no immunities and average Defenses to do.
Keldeo has pretty big flaws, you just have to think about it for a second. You guys are acting like it's the most broken thing ever, and while in terms of raw damage, it can be, but if you look past the calcs, Keldeo's flaws are kind of big, obvious, and easy to take advantage of.
BUT
Keldeo is coming out after Black 2 & White 2 so any number of things can happen for or against Keldeo in that time, so idle speculation about Keldeo is a tad pointless for now since we have no idea what B2 & W2 have in store for us. But for now, I don't foresee Keldeo being entirely unmanageable due to his considerable downfalls. Every team should have 2 or 3 Water-resists anyway since Rain is already
kiiind of a big deal. Rain-Boosted Hydro Pumps existed before Keldeo, even though his is the strongest, I think we can manage.
tl;dr: Power isn't everything. Ask Darmanitan & Chandelure. Unlike Deoxys & Excadrill, Keldeo's too slow and unlike Kyogre & Garchomp, too many Pokemon can just take a hit & roll his ass over due to his average defenses, complete lack of immunities, and inability to change attacks. It sounds crazy, but he's not powerful enough even in Rain. For any Pokemon, 1HKOing the tier and 2HKOing like Keldeo in Rain is a big, big difference.
One thing I have to say even though this is an OU thread; Keldeo is a lot less threatening in DW OU because he does have a good amount of counters there. Ditto rapes CM sets, Raikou's actually good now because of Volt Absorb, Espeon is much more common because of Breloom, Thundurus laughs at Specs Keldeo, and you DARE bring a Water-type without Calm Mind into Contrary Serperior's battlefield?! You're ASKING to be swept if you do that in DW. So to DW players, don't discredit Keldeo because your favorite DW Pokemon beats him. OU doesn't have the luxuries of our weird-ass metagame where Choiced Pokemon not named Chandelure are the worst thing ever.
Choice Specs isn't Keldeo's best set honestly. Calm Mind LO is where it really shines
I lol'd. Use Lefties. Various reasons.
Edit: I just realized something, my damage calcs were done with MODEST Keldeo, not Timid. I'll run the calcs again for Timid and I'll have them posted in a few minutes. My bad~