Well, considering that the only offensive category where Rodriquez distinquished himself is "getting on base', it's relevant that he still trailed Bench.
Bench was an exceptional hitter his whole career, Rodriquez was a very good hitter "for a catcher".
Apples and oranges.
And IMO Bench set the standard for defensive catchers. He doesn't take a back seat to anyone.
Considering that Rodriguez is a better defensive catcher than Bench, Bench certainly does take a back seat to someone. Comparing and contrasting their defensive skills is like comparing two high performance race cars. Both had stellar defense so you would like to have both of them behind the plate but I-Rod stands out from the rest of the pack when it comes to defense.
I think you completely underestimate I-Rod's offensive abilities as well. Even if you take into account Rodriguez's offensive decline around age 36, he still has a career .297 batting average compared to Bench's .267. Bench had more power but I-Rod batting 30 points higher for a career than Bench is something to note and shouldn't be dismissed as "being a good hitter for a catcher".
I'd even argue that Piazza was better than Rodriguez. His overall WAR comes out a bit lower (66.7 to Pudge's 73.4), but he played in roughly 600 fewer games than Pudge. Piazza's WAR/162 comes out to 5.7, even with his horrid defense. The guy was a beast at the plate.
I just can't put Piazza in the same class as Bench, Rodriguez, or Berra simply because Piazza's defense was so horrible. Defense is arguably more important for a catcher than any offensive numbers they can produce for a given team. It is one reason why catchers are automatically given a +1 when calculating WAR (as much as a flawed stat as it is). Looking at pure offensive numbers doesn't do the catchers position justice in my opinion.