People with multiple accounts would then have greater weight. With your proposed system, you just make 100 accounts, age them, and then become very trusted, and start scamming, or ruin/build up other's trust. That would also give people a free pass to scam newbies and jr.members because they couldn't voice their opinions. As it is now, the trust system is not moderated. Allowing staff to alter trust in any way would leave it completely useless. Staff members could then be influenced to do shady things for others or themselves.
As it is now, the people on the default trust list have a vested interest in the forum. First Theymos, those he picked, and from there those that the 2nd layer has picked, etc. If someone does something wrong with the trust system while on the default trust list, it effects the person above them, discouraging abuse, and encouraging those in a position to add others to the trustlist to think carefully about who they add, and act quickly to fix any situation that comes about. If you don't like the default trust system, individuals can remove the Default Trust list from their own account preferences, and replace it with people they personally trust.
I see your point. You actually touched another vital one - multiple accounts, which IMO shouldn't be allowed. People with multiple accounts are already using them to push their opinions, scam signature campaigns and so on. Not allowing newbies and juniors to leave trust ratings wouldn't be a problem, because it would discourage the most obvious spammers, those too lazy to even write a couple of posts.
You're right that it could be abused by people with multiple accounts, but allowing 1 rating per user should be fairly easy. It would only require the software to block multiple ratings by the same user. You should really consider this, even if you're not planning to change the default trust in any way.
As for the people on default trust, I'm not completely sure if people are responsible with their ratings. Just take Mr. CanaryInTheMine. He gave positive trust to a lot of people, who participated in his group buy, so in a way by buying something from him they also bought trust. The trust list is also rarely moderated. Mark Karpeles was on the list for months after his exchange imploded.
I can't speak for others, but Canary added me to his trust list and I've never participated in one of his group buys. I'd suspect the majority of the people there are in a similar situation. There's no reason for Canary to add random group buy participants, as he doesn't have to trust them at all to take their money and send it to Fried cat. They need to trust him.