The problem with making the system only applicable to previously signed transactions is that you would only be able to rate people who agree to it. If you are going to end up giving a negative rating to somebody, why would they agree to let you give them a rating?
The idea is that you would both irrevocably agree to be rated prior to transacting.
The problem is that the system would then be limited to transactions where both parties previously agreed to a rating. What about if we do a transaction, and you rip me off. Can't I then go find you in the OTC system and give you a bad rating?
That's the idea - you're taking a risk by transacting with someone who refuses to agree to be rated.
This wouldn't necessarily exist to the exclusion of the current system, either, but as an augment to it. If someone agreed to a price, then agreed to a pre-authorized rated transaction, and has a good rating already - there's a very good chance they are trustworthy. When those requirements aren't met, you are taking on additional risk.