Let's imagine for a moment that we have some user (not a new multi-account for bounty or scammer) is registering on the forum who wants to learn something new about this wonderful, and at the same time cruel crypto-world, wants to buy BTC for 10-20 bucks in the equivalent of its local fiat currency.
The question is, why this user have to be subscribed to DefaultTrust? Why a new user should trust unknown users from the first second of being on the forum? And what about one of the main principle? -- Dont trust. Verify.
take a look at the market place "digital goods" >
https://bitcointalk.org/index.php?board=93.0i am willing to make a "blind" estimation that the majority " more than 70% of the sellers there are scammers , anyone with an IQ above 0 knows this already.
most of those people are tagged by DT members, this reduces the chance of the new coming users you talk about getting scammed.
the trust system is not perfect at all , giving the fact that you do not know or trust any of the DT members, but it's more like viewing a hotel/restaurant reviews before visiting them, while a lot of the feedback could be wrong/fake you seldom see a very good hotel with more negative reviews than positives, everyone looks for reviews before buying any goods, booking a hotel or a dinner table, you probably do that yourself, while you do not know/trust any of the reviewers their overall feedback usually means something.
TL;DR > the default trust with all the shit it has is differently better than nothing, even if there was a single honest DT member many good souls will be saved.