The goal is to make Baton Pass, as a strategy, more manageable. Establishing a Baton Pass clause will kill the Baton Pass chain archetype (the concept of a chain), and turn the strategy into a 3 Pokemon half-pass/quick-pass team. (Scolipede, Smeargle and Espeon, or a combination of Scolipede, Espeon and Smeargle/Vaporeon/Sylveon/Mr Mime.)
Banning Espeon or Magic Bounce will utterly kill Baton Pass chains, and arguably even half/quick-pass strategies, for fear of Taunt and phasing, etc.
The Baton Pass chain archetype is not uncompetitive or broken in nature. It is a legitimate strategy that should remain intact, and players should be allowed to play a chain style at their own peril. However,
what makes the playstyle uncompetitive is chiefly Scolipede. Scolipede propogates uncompetitiveness on several accounts.
The first, with respectable bulk and speed, and between Protect and Substitute, Scolipede acquires 2-4x Speed effortlessly, while simultaneously boosting Def trivially. Even physical kings are suddenly worthless in the face of an easy x2-3 Def boost. D-Dancing Dragonite or Mega Charizard X are laughably tanked and outpaced at x2, and under the worst circumstances, Scolipede can simply Baton Pass out into a counter.
The second, his sheer versatility and overwhelming support for the chain. While Scolipede commonly runs Iron Defense, he is free to run offensive support with Swords Dance, and can viably sweep on his own between Earthquake and Megahorn, or ensure a better Pokemon can. Toxic Spikes means the chain can dance around and even stall if necessary, and Spikes wears down the opposing team and more allows the chain to OHKO any threat.
The third, his ability to easily set up a second and even third time, and provide second and third chances to facilitate sweeps.
While Scolipede has counters, he can easily avoid checks and counters like hyper offensive presence such as Pinsir and Talonflame. Behind a Substitute, Scolipede can even Baton Pass safely from either of these two offensive threats. Suspecting the typical Scolipede lead, the Baton Pass user can instead lead with Vaporeon, and Acid Armor until Scolipede wishes to enter.
Taunt and phasing, Encore, etc., can stop Scolipede's shenanigans, but due to the nature of Speed Boost, he is welcome to Pass to a counter like Magic Bounce, and reenter at a more opportune time.
By himself, Scolipede takes autonomy from the game, and ensures the entire chain will have speed initiative with no effort, skill and little knowledge of the metagame. Without Scolipede, Baton Pass chains must spend 2-4 extra turns to boost speed via Agility among a special user, making it far more difficult to attain speed initiative, nevermind defensive boosts. (Remember, speed initiative for the entire team means all of Baton Pass' conventional counters are countered.)
Smogon has no desire to blanket ban Baton Pass, or otherwise kill the archetype, and wishes to avoid complex bans unless absolutely necessary. Scolipede would be banned not for brokenness, but uncompetitiveness, and removing autonomy from the game. And his ban would effectively make the Baton Pass playstyle more manageable.
I wish to share some Baton Pass gameplay, under my test account "Its a Genjutsu". I do not claim flawless play, nor do I mean these videos are proof by themselves, but rather the gameplay illustrates Scolipede's general uncompetitiveness and overwhelming support, able to setup and begin a new chain a second or third time, and reducing Baton Pass chains' standard counters into nothing more than whimsical fantasy.
This game demonstrates how Scolipede can set up multiple times with ease, and issue an effortless victory.
http://replay.pokemonshowdown.com/ou-113618218
This gameplay shows how Prankster Taunt Sableeye is no guarantee counter of the chain, nor a counter in general (that is, able to switch in and defeat). This also shows priority like ExtremeSpeed and Sucker Punch failing to "counter" or overcome.
http://replay.pokemonshowdown.com/ou-113232428
These next gameplay videos shows how Scolipede overcomes offensive pressure by the likes of Mega Heracross, Mega Pinsir, Talonflame, etc, and again how easily Scolipede can rebuild the chain. (All mistakes are my own.)
http://replay.pokemonshowdown.com/ou-113611192
http://replay.pokemonshowdown.com/ou-113242095
http://replay.pokemonshowdown.com/ou-113234813
Haze Quagsire isn't a counter; he can't switch in and win, he is worn down or otherwise dies to even unboosted Sylveon, and while he ignores stat-changes, Stored Power's sheer power is capable of destroying him.
http://replay.pokemonshowdown.com/ou-113623014
Scolipede makes it "not fun" and
enables the team to tank x3 Cloyster.
http://replay.pokemonshowdown.com/ou-112747406
Scolipede embarrasses x3 Dragon Dance Mega Charizard X.
http://replay.pokemonshowdown.com/ou-112746268