The neutral ratings is something that can be left is someone is acting suspicious but not acting like they are going to outright scam. Prior to trading with someone, you should check their trust rating, and if you share someone else's suspicions then you may want to trade with an elevated level of caution.
Another reason why a neutral rating may want to be used is if you are on default trust, but do not want to leave outright negative trust, for example if someone is requesting a scammy loan however appears to be doing so because they do not know any better. The neutral trust could be used as a "warning" to be used and if similar behavior is used in the future then an outright negative trust may be warranted
What about the guy who starts yet another Ponzi thread: "guaranteed 200% returns in 24 hours, no limit". He hasn't scammed me, but he's obviously trying to scam. There are noobs in the thread asking questions, and my negative rating could well help them not fall victim to his scam. But he didn't scam me, and won't, because I know it's a scam.
That doesn't seem like a "neutral" case to me.
Negative trust would be appropriate in this case, as they are clearly trying to scam.
The descriptions are really only guidelines as to when it is appropriate to leave various ratings. The trust system is still un-moderated and you still have the right to leave whatever trust ratings you feel appropriate. The added feature is to allow people to add in comments without leaving any actual trust. The use of this feature would only be appropriate in "moderate" events