We've had a bit of a discussion lately about back-up catchers with the trading away of Robinson Chirinos, the signing of Max Ramirez and the head-scratching re-upping of Koyie Hill via the arbitraton process. Reader WISCGRAD put together a spreadsheet of catchers by Baseball Reference's WAR values which does include a defensive component.
Soto and Hill lined up at #17 last year with Soto being the 4th best catcher in just 97 games played if Koyie Hill wasn't dragging him down.
As for that defensive component, here's what BR.com states on their website.
Catcher data looks at stolen bases allowed, caught stealing, errors, pickoffs, passed balls, and wild pitches. I split the data by pitcher handedness (otherwise a catcher will look better if he catches more lefthanders than normal.) Once again, everything is compared to league averages and converted to runs.
Fangraphs also has a WAR value, but last I checked their defensive component for catchers only factors in stolen bases.
What neither site seems to explicitily state is how much catcher reputation is factored in, which would seem easy enough to approximate by just looking at the number of stolen base attempts against that catcher. There are of course other factors, like the pitcher on the mound, how good his move, how quick his delivery is or whether he's a right or a lefty. Without looking at the formulas, that may be factored in, but I don't know for certain. I was just curious myself and if you start at this comment in the Max Ramirez thread, you can follow my math, but here's what I came up with for some of the NL catchers from last year.
Soto -.221% 95 Stolen Base Attempts in 97 games started (.979 attempts per game)
McCann - .300 120 SBA in 129 GS (.93 A/G)
Molina - .485 68 SBA in 130 GS (.523 A/G)
Pudge - .344 64 SBA in 102 GS (.627 A/G)
Martin - .386 70 SBA in 79 GS (.787 A/G)
Hill - .178 45 SBA in 60 GS (.75 A/G)
Soto - .28% 82 SBA in 92 GS (.891 A/G)
Hill - .40% 50 SBA in 69 GS (.724 A/G)
Stolen Base Attempts against McCann and. Soto
2005: .551 A/G (half a season)
Total NL League Stolen Base Attempts
I can't reach any definitive conclusions with such narrow research, but we can see that teams are running on Soto more and more and McCann, a similar catcher better known for his bat than throwing ability, has seen a similar pattern despite the league numbers not really changing much over that time period. It's all a roundabout way of saying it's not too big a surprise why the Cubs would value a back-up catcher that can throw guys out and is better defensively than Soto. Whether Koyie Hill is that guy anymore is the better question.