1: Swihart has not shown any realistic ability to hit better than Leon or Vazquez.
2: Both Vazquez and Leon have produced significant samples of offensive production superior to Swihart's 2015 production.
3: You're talking about a catching tandem that just won the World Series, and the pitchers are effusive in their praise of those catchers' ability.
4: Catcher is one of the two positions, shortstop being the other one, where mediocre offensive production is tolerable as long as the defense is good enough.
5: Between Leon and Vazquez, 70 efforts to steal a base were recorded (and if that seems low considering their combined workload, that's because it hella is) , with 22 of those being caught. That's a 30% success rate, significantly above the league average . So not only were the overall attempts incredibly low (an average year sees somewhre around 120 to 150 attempts to steal on your catchers, and even with Swihart thrown in there were only 89 such attempts on the Boston catching teams which is, again, WAY small, half an attempt per game is TINY) but even those who were brave enough to give it a go, or fast enough to pull it off, were nabbed fairly regularly
In other words Vazquez and Leon did a whizzbang job at controlling the running game, they created a fear factor that scared off a lot of first base coaches and even the guys who felt they were fast enough to make an effort (in other words, we're probably talking mostly about elite runners here) were gunned down at a level above the league average. Combine that with the great work our boys did on the basepaths and it's very easy to understand exactly why we had such a competitive advantage against a lot of other good teams.
Now let's look at Swihart. He played 33 games and saw 19 steal attempts, of which he caught 5. That's a significant increase in the willingness of baserunners to try something on Swihart comprared to other catchers and he only caught baserunners at about an average level. Bottom line here is that while Swihart isn't bad at controlling the running game he's nowhere near as good as either Leon or ESPECIALLY Vazquez, who is a superstar at this aspect of catching defense.
Final analysis baserunners aren't anywhere near as afraid of Swihart as they are of L and V and will try his arm more, resulting in increased scoring chances for the opposing team than if we stick to L and V, regardless of other factors of offense or defense. I'm reluctant to surrender an advantage of that magnitude in the name of a single hitter hitting a few extra hundred OPS points, even if Swihart would definitely hit it might not be worth it -- but to surrender an advantage this huge for merely the CHANCE of a slight offensive uptick -- on a team whose offense is already at juggernaut levels -- is patently ridiculous.