Providing access to the value
What are you going to do with the value? Remember, that's an Object, not a String or anything else.
Anyway, its available in MessageInterpolator.Context.getValidatedValue()
A custom interpolator can make use of it and burn itself :)
The apps I work on usually have error messages like: "Date yyyy-mm-dd is not a working day". This is useful because the user can see what the app got, instead of what he typed (so if there's some conversion and translation going he can see something weird going on).
I ended up writing a custom interpolator to do this (actually we support arbitrary EL in the message parameter, "this" binds to getValidatedValue()), but IME it's a common requirement, so it's a good candidate to be in the spec.