Here's what I thought....
With no failures, reversers are never taken into considerations when calculating distances. However, with a failure and a wet/contaminated rwy, then reversers must be considered in the distances.
Sounds like some are trying to push the limits a little by reducing the landing distance calculations by all means...perf calculations have evolved from the industry's experience and accidents/incidents records...why not use that wisdom and stick to well accepted policies on how to calculate distances? Even if you find a piece of law that allows you to use reversers in all calculations, it won't really help you once you're in the dirt at the far end of the rwy!