HADOOP_CLASSPATH tends to be used to add to bin/hadoop's classpath. Because of the way the comment is written, administrator's who customize hadoop-env.sh often inadvertently disable user's abilities to use it, by not including the present value of the variable.
I propose we change the commented out suggestion code to include the present value.
# Extra Java CLASSPATH elements. Optional. -# export HADOOP_CLASSPATH= +# export HADOOP_CLASSPATH="<extra_entries>:$HADOOP_CLASSPATH"