I've often written the same thing to newbies when they ask questions like this, but I've come to see it from their point of view as of late.  It might not be obvious to a new member which section to search in or if there's even been a discussion about what they want to know--so at least in this case, I don't fault OP for asking about DT and the use of signatures.

It isn't, but I thought most Hero-->Legendary members did put at least something in their signature space, even if it's not advertising.  Last time I saw threads with signatures enabled, it seemed like everyone--not just those two ranks--was in a sig campaign or at least had a message written in there.  If that isn't the case anymore, then I have no idea why that'd be true.  I have a feeling it isn't, though.  Not with all the bounties and campaigns still running.

And as far as the default trust system goes....I don't care to write much about that one.  OP would do well to check out those links provided by igehhh.  Things used to be a whole lot different a few years ago, and members even on DT2 were fairly well-known.  Nowadays I have no clue who's on DT1 or 2 and if I looked at the list I bet I wouldn't recognize 40% of the members.