*IF* you were to ever find yourself on the DT *all* of your previous trust, good, bad and indifferent will suddenly be upgraded. Dozens or even hundreds of UID's would then automatically bump to red not just on your trust but every-ones.
I'll echo this sentiment and add that when I got put on DT the first time, I'd left tons of negs for account sellers. When I got on DT2, my inbox was instantly flooded with confused account sellers and requests that I remove the negs I left, sometimes months previously. It was kind of amusing.
So yes, there's definitely a good reason why you should leave feedback for scammers when you find them. Don't rely on DT members to do it, because it may not happen.