The main thing about negative trust is that you'll have a very hard time joining a signature campaign, since most campaigns have rules about not accepting members with negs from DT1/2 members. Projects don't want members whose trust shows up as red advertising their ICO or whatever it is they're promoting--and that makes sense.
The other effect is that if you have negative trust it's harder to do deals on the forum, since many people are reluctant to trade with red-trusted folks. On the other hand, it's not exactly the kiss of death as far as trading is concerned. You see people getting scammed here all the time by members who already had red trust, so it's obvious many people don't even look at people's feedback.
I was thinking that would be another way to lessen possible spammers
Huh? Negative trust doesn't have much to do with spamming. That's more a merit issue than a trust one. I think you might be getting those two things confused.