Dugtrio has really poor stats to do some work with his abilities. With the loss of perma-weather he saw little to no usage in gen VI, not being able to open up the way for a Chlorophyll Venu or a Volcarona' sweep anymore. He is somehow getting usage right now with M-Pidgeot, but Pidgeot is easily revenge killed, doesn't fair to well in the OU meta and doesn't usually represents a win condition for his team. Dug is decent at what is does, removing Heatran, some weakened Tyranitar, and generally low health mons (Keldeo, Terrakion, Metagross, ...). But that's really all it does. It is really to frail to pose himself as a huge threat if the opponent is not carrying Heatran or what else.
Magnezone's Magnet Pull helps some offensive teams to break through the steel type counters his physical sweeper have to deal with. It is really helpful on the ladder, where Ferrothorn is on most balanced teams and Skarmory runs rampant on stall teams. Still, it doesn't trap Slowbro, Chestnaught, Rhyperior, Clefable, Gliscor, Landorus-T and other decent walls, who can still put work against the offensive threats of the magnezone's team. Skarmory and Ferro might even carry Shed Shell, but different offensive teams adapted to that using some "knock off lures", see Mamoswine, Clefable, Landorus and everything else that can run Knock Off, really. Some SpDef Ferrothorns can even surprise the non-specs HP Fire Magnezone and kill him with Leech Seed + Protect, while the specs variant are usually only deadweight against offensive teams. Magneton should see no usage if Greninja gets banned, so I won't discuss that.
The last one, Gothitelle, is just a "superior Magnezone" in almost every aspect. Shadow Tag is probably the best ability in the game. You trap something, bar ghosts, and remove it from the match. Are you running offense and Gothitelle? Use a specs variant to trap the counters to your mons and let them clean their way through. Do you have troubles against stall? Try the scarfed CM-Rest set with Trick, which helps you to remove every single defensive threats your opponent is running. Are you using stall? Use that last set again, and trap&trick the few threats that can 6-0 you (for example, RD TG Manaphy, CM Clefable, ...). You just realize what does your team need, and run the most successful set against the type of team you are weak to. Maybe I am exposing Gothitelle as the best mon in the game, while it obviously have sub-par stats and many flaws. While the specs variant is super useful to eliminate defensive threats in balanced teams, it is completely deadweight against offense, and the scarf set is useful only at trapping opposing Keldeos and weakening them (uninvested Psychic does 70ish% to it) or crippling mons tricking the scarf away.
Even running Pursuit Tyranitar doesn't stop Gothitelle from doing his work. I want to make some pratical examples.
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I was running a Clef-Tyranitar team on the ladder in the last few days. I met a few Sableye/Gothitelle teams. The pressure I had on my shoulders was just as high as the pressure my opponent had. In a possible Clef v Sableye matchup, he can go into his Gothitelle and trap my Clefable, or he can predict my Tyranitar switch and will-o-wisp it with Sableye, crippling it for the rest of the match. Running Pursuit Tyranitar is probably the best answer balanced teams can bring to the table, and even then, you are gambling on a 50/50 on whether will you lose your Clefable or not. And this might be the best scenario for the trapper's opponent.
And if you are not running a pursuit user, pressure will only be on your side. Do you have Ferro out and predict the opposing Magnezone and switch out, gaining momentum but risking him to stay in and attack your possible switch-in? Do you double into something to avoid seeing your stallbreaker trapped by Gothitelle, getting more hazards damage just to see him predicting that and not switching into his trapper? You have a whole lot to lose, and the trapper's user doesn't. That's why I don't think trapping abilities affect in a positive way the metagame. They are uncompetitive, and don't represent an ideal way of how the metagame should be, in my honest opinion. Shadow Tag is just the stupidest and best of them all.