More and more research shows that display ads and in more general online branding is good for conversion.

If you only measure clicks, or even worse last-click, you miss the point of branding completely. People buy more easily when they know your brand or after seeing your display ads.

Check this article about engagement mapping from Microsoft advertising.

eMarketer recently published an article:  eMarketer seven strategies. In this article (starting from page 15) eMarketer shows even the impact of display advertising on Offline sales.

So people learn about your brand online and finally buy offline.

ps. I have business connections with eMarketer