Well, aside from what's already was mentioned it would be unnecessary work to maintain such a list of "trusted users" amongst other things making it a less-than-ideal way to go about it.
The exact same thing you're suggesting could be achieved by simply rephrasing the rule to suggest that one should not excessively post one-word responses rather than banning it altogether without having to use some complicated system where some users are allowed to and others are not.
However, that is still rather besides the point since, as mentioned, there good are reasons for the rule being there and for it being enforced rather strictly as opposed to only restricting excessive such.
Once again, it should be easy enough to add another word or two to make your post look nicer; For example "Agreed" could rather be written as either "I agree" or "Yep, I agree completely! That's a great idea!" and neither of those would break the rules yet (especially in the case of the latter) look nicer to the person reading it.