I've been a prosecutor, been a defense attorney and been on both a Grand Jury for a month and on a trial jury and I've been a pro-tem Judge.  For a total of ten-years experience. 

Other than for your friends, the system seems to be working to me.  

If they don't get arrested again, (not all that likely in my personal observation and experience) then society has been served.  

Oh, by the way.

In case they do get arrested again, the fact that they were convicted can be admitted in court to impugn their credibility before that jury should they want to testify.