I think part of the problem is much of what they learn is specific to the hospital they're doing residency at. Besides much of what they learn not being tangibly useful in a clinical setting, nearly the rest of what they learn (Policy, protocols, p&t meetings, research, and even kinetics) are all subject to how that hospital decides to do things.
Sure, it makes them an excellent candidate for the hospital that did the residency in, but I don't believe it's worth more "experience" elsewhere. Even then it's not a sure thing; one of the worst pharmacists at my hospital was residency trained in-house.
All we know is that they're residency trained and maybe the reputation of the residency.
What we don't know is how well they were trained or performed.
Maybe if there was a grading system in place or test scores from the bcps? Granted, normal employees aren't expected to show their previous evaluations to future employers. But if employers are mandating residency and/or equating it to multiple years of experience, it seems fair.