I don't think people are necessarily against preventive feedback as you call it, but just feel like tagging people for a scam that hasn't happened yet is an incorrect use of the trust system. Been saying for a while that we need 2 different trust systems, 1 for reputation and 1 for trading, or we just need to adapt the current feedback system and let it be used for more than trades.
People leave feedback for damn near anything anyways. May as well use it as a hybrid system and combine trade and reputation as 1 system.
There are times when negative feedback before a scam is invaluable. Recently, a trusted member tried to get a lot of coin based on collateral that she had private funds coming in this spring. She may have succeeded if I had not warned people, and I can always remove my feedback and make arrears should she actually show proof of funds. But there were so many contradictions in her story she has lied at least once already.
A "trade" trust system would be controlled by the scammers with most stolen coin. Sell/give/trade items (or pay aa few people in a few cities to do it on his behalf. Then he reciprotate the trust (even though he had no risk) and corrupts the default trust.
I am always thinking about new algorithms - I'm going to play with a few (like I did with BPIP) and see if we can come up with a less abusable system. Since I don't play the DT games, I'll eventually be knocked off it.