Thanks for pointing that out :).
OK, first things first, you would agree that the user of evasion would have to be properly bulky, like a Gliscor, Skarm, Dusknoir, or whatever. It also should avoid being weak to fighting (hence why Umbreon, Heatran, and T-Tar would not do good jobs). The thing is with crits, if someone is using a pokemon with enough initial power, it's likley to 1HKO the pokemon, as it utterly avoids the stat-boosts and has double power. Thus, if you managed to land a crit on a pokemon using such, it would be gg, as most of those pokemon will be attempteing to do it with a supereffective, or at least neutrally effective move. With evasion, you have to worry less about these random crits as you have that extra chance to dodge them. Considering that you are using a defensive pokemon that has quite the bit of chance to survive one supereffective move, your pokemon still won't be taken down easily, even if the opponent hits.
I have to agree here. However, there are still common scenarios in which bulky pokemon can be OHKOed, excepting perhaps when talking about Ubers and Cresselia (the latter has near no offensive cabilites and even with +6 CM, she is walled by Blissey. For other bulky pokemon, Gliscor hates Ice Beam and even Ice Punch, Skarmory has Electric and Fire problems, Dusknoir hates Tyranitar and the list goes on.
Erm... please only Compare it to Cosmic Power. Only Cosmic Power protects you from both sides of the spectrum in one move slot. The most defensive use of Cosmic Power... is Claydol. The most powerful with a one spot recovery move is... Clefable (which actually is really good in doubles for Follow Me-ing if you get the Cosmic Powers up).
I agree though, it would be just as difficult to get +6 evasion as +6 Cosmic Power. However, I think the crits would make Cosmic Power to risky. About the type coverage though... have you ever thought about using moves that force them to switch on entry hazards, or what about running this during a Sandstorm. The entry hazards add up, and Double Team has quite a bit of PP (though most of it is useless). Imagine facing a whole team of double teamers, eventually, you would die of either running out of PP or switching on entry hazards too much (you cannot Heal because of Taunt).
I admit that Cosmic Power has very few good users. However, I think we can all agree that DT is most powerful when used with Baton Pass.
If the DT user manages to get +6 Evasion and has Taunt, what happens then? Sweeping attempts and residual damage will be just too hard to pull off. Entry hazards mean little against Wish, and Sandstorm is negated by Leftovers. Your DT + Taunt + Roar suggestion is ruined if Rapid Spin manages to hit the DT user, and that's a 50% chance after 3 DTs. Of course, this argument doesn't apply for Dusknoir, but as you said, he's a too slow Taunt user.
I agree, Dusknoir is a bit too slow for the ideal Taunt. Try looking for something a bit faster.
Strickly speaking, the best users of DT are bulky. EVing them in Speed will remove some of their bulk and make the challenge of pulling off +6 Evasion even harder. And even if they are EVed for speed, there will be still plenty of faster pokemon that can know Taunt.