Their "strong", "moderate" and "at risk" categories are too broad and vague to make an accurate assessment of how our changes are working. And to just have it run on Google alone to see the rate defeats the purpose of advertising on Google, to get lots of targeted traffic from them AND their partners.
With a CTR we will know exactly how our keywords are doing on Google only by seeing the rate either increase or decrease with the changes we make.
This will benefit both advertiser and the user as ads will be more relevant and we as advertisers can maximize our campaigns (which benefits Google).