EVERY DEFAULT USER CREATED BY ORACLE HAS  A PURPOSE ( which is ofcourse optional) except for sample/demo schema's like SCOTT.
There should no harm in locking SCOTT.
But, dont drop or lock the following accounts,
UNLESS u are clear that these options are NOT USED.

Oracle adds the OUTLN user schema to support Plan Stability. The OUTLN user  acts as a place to centrally manage metadata associated with stored outlines. 
DBSNMP                    if Enterprise Manager Intelligent Agent is installed    
TRACESVR                  if Enterprise Manager is installed   
AURORA$ORB$UNAUTHENTICATED 
   
AURORA$JIS$UTILITY$       if Oracle Servlet Engine (OSE) is installed   
OSE$HTTP$ADMIN
/
MDSYS                     if Oracle Spatial option is installed    
ORDSYS                    if interMedia Audio option is installed
ORDPLUGINS                if interMedia Audio option is installed     
CTXSYS                    if Oracle Text option is installed    
REPADMIN                  if Replication Option is installed 
LBACSYS                   if Oracle Label Security option is installed 
ODM                       if Oracle Data Mining option is installed   
OLAPSYS                   if OLAP option is installed