Obviously this is harmful to users because the implication is that the technique also requires SSL stripping, or trusting invalid root certificates like we saw with Superfish. It's also harmful to advertisers and ad networks because it pollutes tracking data and makes it hard to determine click fraud.
But let's not kid ourselves. Google does not care about the user. They simply have no need for ad injectors because they already have far superior methods of tracking us and invasively advertising to us (reading our email, watching our GPS location, knowing when we are home, what videos we watch, etc.) To google this is just a nuisance and they get some free PR for standing up to it along with a respected academic institution. Yay, google! Protector of users!
But wait. Isn't this exactly what Verizon, ATT, and Comcast are all doing? Verizon was modifying HTTP headers during the summer. ATT charges users not to inject tracking into packets. Comcast injects HTML into xfinitiwifi connections. How is this any different? Sure, tracking headers do not manifest themselves in annoying pop up ads, but they are still messing with user requests and have almost as many security implications.
If Google is going to take a stand against ad injectors, they need to take a stand against all packet injection. These scammy popups are just the bottom of the totem pole. If they could get away with what the big telecoms are doing, they would obvioisly do that instead.