My philosophy on link building in general is that you should only pursue an individual link opportunity if there is intrinsic value in being present on the site, and don’t do it JUST for link building. This goes for banner ads and text links alike.

If you are responsible for generating leads, then pursue a link if you think it will help with this goal (e.g. people will see the banner/link and click on it to come to your site and convert).

If you are responsible for generating traffic only (this would be too bad because this means you are probably not effectively targeting profit goals) – then you would care about the CTR of the banner/link.

If you are responsible for generating ad revenue on your own site – you would want a combination of a high CTR & engagement on your site once people arrive (e.g. high pageview counts, low bounce rates, viewing the most valuable content, etc.)

If you are responsible for branding – then you would look at a combination of impressions for the ad/banner/link & visits to the site + engagement on your site after arrival).

To answer your question directly – not all banners are created equal. It will depend on the quality of the site you are placing the ads on, the creative you use for the banners, etc.

That said, I personally encourage clients to pursue banner advertising as one of their last options.

Hope this helps!