The sys_pattern was created to differentiate between file appender and syslog appender:

:pattern: "%d [%.1l|%.3c|%.8X{request}] m\n" 
  :sys_pattern: "[.1l|%.3c|%.8X{request}] %m\n"

It does not make sense to have the same defaults for both, syslog should really be just %m (message itself), all the rest can be added in syslog configuration when needed.