As the only way to consistently pass speed to a teammate, this specific combination turns otherwise manageable hard hitters into ridiculously effective sweepers. In a metagame without Eviolite, the number of checks to, for example, a boosted Clamperl with a set of Surf/Ice Beam/HP Grass, is limited to a few otherwise unviable or at least not very good sets, such as specially defensive variants of Lotad and Surskit. Ledyba has a great deal of special bulk, and Spinarak lacks double weaknesses to take advantage of, so it isn't difficult for them to find the opportunity to set up. Key recipients include Clamperl, Cubone, and Drowzee, but there are other effective choices, although they are generally somewhat less broken. This decent variety makes it difficult to predict what will be coming in, especially with the lack of team preview, so trying to OHKO a recipient as soon as it enters the field is often not feasible.
We have chosen not to ban the broken recipients, as they have notable but less disruptive niches as well (Cubone is B+, slam clam is C+, and Drowzee is C-), or the passers, because they have their unique, albeit not all that viable niches. Agility and Baton Pass have significant uses outside of being used in conjunction, so neither will be banned individually.
