I don't think Google will ever offer this feature - it would officially show advertisers just how much money they are wasting due to all the crappy sites in the content network. It will just flat out make Google look bad.
With Site Targeting at least they can blame the crappy results on you since you picked the sites LOL.
It would also cost them a TON of profits if they made it easy to see which sites are sending crappy traffic AND allow you to block them. As it is now they can pretend they are trying to be helpful by letting you exclude sites, they just don't help you figure out which ones to block.
In my experience most sites would need to block 50% or more of the content network sites that send them traffic. With the content network being enabled by default, and most advertisers probably not changing that, you are talking about tons of lost revenue if they made it too easy.
Average joe advertiser believes Google when they say they have sophosticated targeting systems blah blah blah and would be shocked if they really knew where a lot of their ads were being shown. I would estimate that over half of the content network sites shouldn't even be allowed in the network, based on Google's own policies (that they apparently don't enforce).
Don't get me wrong, there is a ton of quality traffic to be had from the content network ... you just need a way to figure out which is which and block the bad traffic. I just don't see Google helping with this ...