We wouldn’t necessarily need to include the unreliable data, we just need a way to distinguish between “reliably unspecified” and “specified but unreliable”. As @uranusjr points out, this is particularly important with repeated-use fields like Requires-Dist where the absence of the field should indicate that no dependencies were specified.
If your suggestion was that we would only include fields that are required to be reliable in the metadata file, then that changes the discussion drastically, and changes it into something I would consider much less useful. As I mentioned in my response to Donald, I foresee a transition from most metadata being “unreliable” to most metadata being reliable as we improve ways of specifying static metadata and detecting statically-defined metadata in a setup.py. Having a marker for which fields have undergone that transition will make it much easier for pip and other tools to get significant incremental improvements as adoption increases.