A security risk in using pphbb instead of phpbb? I can't see why.
Installing in /opt isn't really an e-smith-specific thing, but was recommended a while back by one of the developers (I think by Charlie) as a security measure. If you put the app in an ibay, any user on the LAN can view the PHP source for any of the files. By putting it in /opt, you've restricted that. There may be other reasons as well.