When talking to vendors several have said they do not pay attention to CTR (click-through-rate) as the CPA (cost-per-acquisition) is what matters.
There needs to be a balance between all aspects of a measurable campaign from CTR (click-through-rate), CPC (cost-per-click), CAR (continued-action-rate), and CPA (cost-per-acquisition). It's a funnel. In order to help improve the CPA, improve the CTR. If your message is being put in front of the right people more often more people will click through and ultimately convert.
Also, in the first couple of weeks it's not clear as to what is working best because you may not yet be accounting for delayed conversions or view-through conversions. CTR allows you to optimize your creative and messaging immediately. You can initially see what creative is working best where and optimize to have them delivered there more often.
In ClickZ's article Is the Click Dead or Just Misunderstood? they give two examples about where the click matters in advertising, the Like button on Facebook and in Google Ads. 93% of Facebook users click the Like button and of that 51% are more likely to purchase the product. For Google Ads and Search there was a study done that showed the importance of organic search and paid search and how there was brand recall across both. This is very similar to view and click conversions in display advertising showing that just as important are the impressions as the click itself.
Impressions and clicks are both needed to get to the CTR and if you measure CTR you'll get an initial response on how your creative is performing. If optimized towards putting which creative performs best where on targeted impressions there is a strong correlation between CTR and CAR. When working on tech client I was optimizing the CTR against targeted placements, optimizing the creative towards the better performing click-through-rates, and the CAR improved over the length of a campaign, opposed to when I didn't optimize the creative against those targeted placements and just let the campaign run.
CTR does not tell you that your campaign was or was not successful. At the end of the day success is measured against your conversions and what ultimately filters into dollars or an ROI. But, to get there all aspects of the campaign need to be paid attention to and optimized against, even the CTR.
