PacBSD is just FreeBSD using arch packages (but compiled for FreeBSD), the benefits of using anything FreeBSD based are few, and mostly sentimental, like joining a boinc team. BoincOS point is to have a quick to drop boinc dedicated OS, and if you are computing for anything that has apps avaliable for FreeBSD it is going to run very similarly than on linux.
Its most interesting use is for anyone having a NAS or Firewall computer running FreeBSD (extremely common, ZFS and pf are very superior), which only works in small sprints to use their free resources this way. (not like anyone couldnt just install boinc in their current setups, but having things pre-set helps the lazy, and im lazy.