It's not a bad ability by any means, in fact it'd be really nice if more viable things got it.
But then you realize that Gothitelle has Shadow Tag.
It'd be kind of like if something got both Guts and Pure Power. Guts is normally a great ability but why the hell would you use it on that mon?
Well, for Guts versus Pure Power they both do the same thing (boost Physical attacks) but Pure Power does it better. Competitive at least does something completely different from Shadow Tag. It might even be viable if Gothitelle weren't horribly fragile. It does not appreciate Utility Scizor or Mandibuzz.
More to the point:
http://pokemonshowdown.com/replay/ou-90871842 — Let's count the silly things in this replay:
>Sniper Kingdra with Muddy Water instead of Hydro Pump or Surf. Unfortunately for it, I Knock Off its Razor Claw first turn. It then decides to stay in on Mega Gardevoir and get owned by Hyper Voice.
>Sitrus Berry Talonflame that goes straight for the Flare Blitz when confronted by an Aegislash.
>Me underestimating Cloyster's Speed and trying to Taunt it to block Shell Smash.
>Cloyster using Rock Blast on Aegislash instead of Icicle Spear.
>
Air Balloon Blastoise with Earthquake.
>My opponent deciding their Physical Dragonite with no Flying STAB is a good switch-in for my Iron Defence-Passing Scolipede.
>My opponent switching out Dragonite after setting up a Dragon Dance, just so their Blastoise can be OHKOd by Zen Headbutt. (Although, it could be they were trying to decide what to sack to get Talonflame in.)
But the best part is my opponent's reaction to Celebi's raw damage output.
One time I fought a Scarfed Machamp. My opponent Baton Passed it a Sub, which made me automatically assume it had Focus Punch, but I never saw Focus Punch the whole match. What I did see was Thunder Punch/Earthquake/Stone Edge, which meant Celebi tanked the whole set and net me the last two KOs of the game. (Even Stone Edge, the only move he has that deals neutral damage, is a 3HKO.) They even crit the Stone Edge, which accomplished nothing more than forcing Celebi to die of Life Orb recoil. When Machamp was their last mon, and I still had three left.
http://pokemonshowdown.com/replay/ou-90711397 — I found someone using an Eeveelution team. What they apparently don't realize is that Moonblast's Sp. Atk drop forces out my Special-attacking Aegislash. And they feel that it is a good idea to forfeit as a result of this.
http://pokemonshowdown.com/replay/ou-90859692 — Muscle Band and Wise Glasses? Well, I suppose extra damage output is good, but aren't there more optimal items to use to accomplish this? Also, Mega Gardevoir sweep.
Not as silly, but one time I lost a match because my opponent's Swords Dance-Passing Scolipede had 252 HP / 252+ Def and ran Sitrus Berry instead of Black Sludge. This enabled it to survive Azumarill's +6 Aqua Jet with 10% HP remaining, and then avoid being revenged by Celebi's Life Orb Sucker Punch. The Scolipede itself KOd both Azumarill and Celebi (with X-Scissor, not Megahorn), then Baton Passed to one of its friends who took out another mon before going down. My own Scolipede then missed it with Megahorn, allowing it to set up again. It then Baton Passed to something that swept the other half of my team.
What. The. Hell.