Actually, none of this is true. Where to start?
Ok, Skarm normally does run Max Def in this meta, because with all the hard hitters (and its low Sp Def), it can't really afford to not go Max Def (unless it is not your main physical wall, in which you would have a different way to kill Excadrill anyway). Running Sp Def on Skarm would basically limit its usefulness, as even with Max Sp Def it's not that specially bulky. So your Sp Def point is pretty much moot.
Secondly, Adamant Balloon Excadrill fails to 3HKO Skarm with a +2 Rock Slide, assuming Skarm has Lefties. So actually, Skarm could Brave Bird, Roost a couple times, Brave Bird, again, and repeat the whole process. Balloon Exca loses.
Adamant LO Excadrill fails to 2HKO Skarm with a +2 Rock Slide. So Skarm could Whirlwind it out to rack up Spikes damage. And then finish it off with Brave Bird later on.
Third, Skarm very rarely runs Taunt in this meta, because it simply needs every single one of its other moves. If it could have a fifth move, yes, it would use Taunt, but losing any of its four standard moves would hurt it more than help. And that's not even saying that using Taunt makes Skarm itself complete Taunt-bait. Somehow, I wonder why hurting Skarm makes Taunt "clearly better". Skarm is best off setting up along side Ferrothorn, or switching out. If you think Brave Bird is only for Excadrill, I legitimately question your knowledge of the metagame.
To say Excadrill has no fear of retaliation is foolish. To be a little harsh, saying that Skarm doesn't counter Exca almost makes me think that you just have an Excadrill-weak team. A lot of times (i.e. when Excadrill uses Balloon), a crit or flinch won't even mean the end for Skarmory.
What? Having a Skarmory on your team in this metagame forces you to make a lot of concessions, since you give things like Gliscor, Thunderus, Heatran, and Rotom-W a free switch. Defensively, you'd need at least two parts of a FWG core and a decent way of dealing with Thunderus -- which is kind of hard. Latias is an alternative possibility for a defensive pokemon, but playing with Latias in the early to midgame against Tyranitar is always a big risk. Moreover, unless you're willing to run Shed Shell, which is an awful idea, you can't really pair it with Nattorei or your team is too weak to Magnezone. The end result of all this is that good teams with Skarmory almost invariably rely on Skarmory to handle threats like Gengar, phaze Reuniclus in a pinch, to Brave Bird Celebi, to take the occasion Draco Meteor, etc. All of which require SpD. Moreover, without SpD, Garchomp can just run Fire Blast to get through it. Since most teams with Skarmory rely on it to handle Garchomp, it needs at least some SpD to be able to later Roost at vulnerabilities (Nattorei, Bronzong) in the opponent's team. That's why, even though Skarmory is by miles the best answer to SubChomp and an okay check to Excadrill and Reuniclus, exceedingly few people use it at the top of the ladder: compensating for the massive hole it puts in your team costs too much. Because of that, Skarmory is very often weak and, outside of full stall, rarely physically defensive. It's the same reason Skarm was always specially defensive last gen, only exacerbated. I just finished laddering to reqs a few minutes ago and I only saw 3 other players using Skarmory, as a metric. To put that into context, I saw more players using Ononokusu than Skarmory. All of them on teams more defensively-based than mine, at least one of which with significant special investment. I don't think Careful Skarmorys have any business in this metagame, but I think that, optimally, the EVs should be split between SpD and Def in some form. If you're at the top of the ladder with a fully physical Skarmory, I guess you're a better player than I am? I can vaguely see it being possible if paired with Celebi, but I can't see it being as effective as specially defensive variant.
Those calcs all assume that Skarmory is at full health. And, I mean, it really isn't. Skarmory has to switch into Garchomp and Ononokusu and is often the main switch against Landros. It really needs to set up against Jirachi and Ferrothorn too, leaving it open to paralysis. I mean, assuming you do phaze Excadrill the first time, you're going to have to look closely for an opportunity to heal the 30%. And, if you don't, you're risking that it SDs a second time every time you Roost or Brave Bird, meaning you'll be even weaker -- if not dead -- by the time you actually get it out of there.
And, again, what? In a metagame where the number one pokemon is a grass/steel type without Rapid Spin that learns Spikes, Leech Seed, Thunder Wave, and Stealth Rock, Brave Bird is the clearly superior option? In a metagame where two of the most important switches into Skarmory, Gliscor and Thunderus, set up +2 Atk, Brave Bird is the superior option? I mean, Brave Bird definitely has its uses outside of Excadrill, like hitting Celebi or breaking Heatran's Balloon or just hitting stuff hard, but my point is that Taunt is
nonexistent when it should be a serious, if not more prevalent, option.
I don't call 24% when I can SD or Rock Slide for 30-40% retaliation, mostly... My point isn't that Skarmory doesn't check Excadrill, or that it isn't good against Excadrill, it's that with a small amount of hax and proper support Excadrill just kills it. In the broader perspective of the metagame and the play, Skarmory's a great check to Excadrill; but, it's far from a counter. I'm not trying to pick a fight, I just think that Rock Slide flinches beating Skarmory is a really valid point.
And this is assuming your opponent doesn't just have a Magnezone on his team. Teams with Skarmory can't really afford a second physical wall at all, so it's not even theorycraft: by having Skarmory, your team is automatically much weaker to a solid section of the metagame.
I don't know, maybe I'm just being stubborn because that's the way I constructed the metagame.
Yeah, Excadrill does need a coverage move. If it doesn't use one, it loses to any Ground-immune/resistant poke who is not weak to Rock. Celebi, for example. Lati@s, for example. Bronzong, for example. Hell, even Reuniclus can tank a +2 EQ and then OHKO with Focus Blast. Do you want me to list more?
By trying to beat Gliscor, Excadrill causes itself to lose against so many more Pokemon that it is simply never worth it. It's not as effective as X-Scissor (arguably the best coverage move) and it's not as effective as Rapid Spin. MixDrill might get a few sweeps off the surprise factor, but it's just not as good as the standard Excadrill moves.
Reuniclus takes more damage from a +2 EQ than it does a +2 X-Scissor. Bronzong walls it no matter what. Most good players run Frustration for the neutral damage against Rotom-W and Gliscor anyway, so it's not like Celebi's on most people's minds. Or that common, even. Lati@s is a valid point, I guess, but it's a lot easier to get rid of those than Gliscor. I mean, I'm not saying HP Ice should be its main set or anything, but it's not losing much at all from running it. I'm not advocating it, but I strongly disagree that it's invalid...