I’ve left a few people +ve trust without a reference.
I've done likewise, although if I'm leaving a negative for someone I usually feel the need to leave a reference link, or else it's kind of meaningless. When I look at someone's trust page and see a bunch of negs without reference (especially if they're untrusted feedback), I tend to ignore them. The reason for that is because there's so much retaliatory and malicious feedback being handed out that I wouldn't necessarily not do business with a member just because someone called them a scammer with no evidence to back it up.
As far as leaving positive feedback, I'm extremely conservative about doing so since I got added to DT, but I've given positive trust to members who I think are trustworthy even though I've never transacted with them and thus have no reference link to provide. I don't think that's a problem, since if they turn out to be untrustworthy the feedback can always be removed. That's basically what happened with aTriz, although I think in his case I'd left him positive trust for being an excellent campaign manager and did provide a reference.
I have had negs for simply not fighting against a scam someone told me I should help them fight because I was busy fighting larger scams and the perp had vanished from the board so there was nothing you could really do
I've been negged by turds for reasons even more trivial than that, and if you look at some of them they actually claimed I scammed them for X amount of bitcoin or tried to extort them or whatever. Since they never provide references (and the fact that none of those things ever happened in the first place), there's no way for anyone to verify the negative trust.
In any case, I think it's much more important that references be left when leaving negs than positives, but there can be and is abuse occurring when no reference link is provided. I've seen alt accounts giving each other positive trust before, and that's just one example of what could happen.
Since I am not DT, my trust ratings are more like a note to myself if I encounter that user again.
I think that's completely fine. I also think more people should create their own trust lists and not rely on the DT system. I have a custom trust list and the default trust system isn't a part of it.