Validation has been implemented as method validate AND it's fired for every metadata creation (aka as part of the constructor). I think it's better to have valid metadata since we are working with immutable object at metadata level (at least in 99% of cases). Of course the method validate is public and can be fired by metadata's client too in case of metadata forced modification (forceXXX methods). Opinions?
The validator is also available as a standalone tool, and have Ant and Maven integration. Having the validator as part of our deployment chain is to ensure that people are actually using it before deploying to our container.
So all the triggered rules should be collected and summaried in the generated report.
Of course make this fine grained check would need some extra analysis and some extra documentation, but I think we would get great benefits.
Yes, it would be nice to have a very fine gained analysis of the deployments. However, I think it is of lower priority than some of our other tasks atm. If we have some sort of framework for these validation rules it would be a good task for a new contributor.