Well they may look unused in the builtin tools. but might be leveraged by user's process. I think that this per step statistic metric is a good feature. This can be safely moved into properties by prefixing with the step name (which is unique by design)

self.setProperty(self.name+ '.warnings', self.warnings, "sphinx")
self.setProperty('warnings', self.getProperty('warnings') + self.warnings, "sphinx")

this can be factorized in setStatistics method or process.buildstep