Bare access of parameters fails the first time a job with parameters is run.  It also fails when parameter names change.  This produces inconsistent pipeline results depending on when the pipeline is run (first run or not).  

A recent PR ( https://github.com/jenkins-infra/jenkins.io/pull/746 ) was filed to change the documentation to say code should use params.NAME instead of bare NAME.  There have also been multiple bugs filed by users getting bit by this: 

See https://issues.jenkins-ci.org/browse/JENKINS-40631 among many others.

Given the confusion and error prone nature of bare access, we should make bare access obsolete and move toward removing it - all parameter access should be via "params.ParameterName" path.