The utility of the trust system is quite obvious. I'm not sure if your question is real.
Without any trust system at all, scammers could keep scamming without restrictions, without even using a new account. Without any trust the only thing newbies would see is rank, so a scammer can get a Legendary account and scam a lot of users.
Without any default trust list at all, users would need to set up their own list. This makes perfect sense for experienced users who know whom to trust. But newbies don't know anybody so they would either add anyone to their list (randomly or Legendaries assuming that means the user can be trusted) or nobody. At least a list of suggestions of trusted users is required.

In fact, default trust should be shown even to non-registered users. I hope theymos does that soon.