It's already been stated by admin Devon that ads don't generate enough revenue to be worth implementing to allow as a method for refilling the energy bar. However, within the planned feature section admin David has stated that there are plans to allow F2P players to earn dragon scales. As scales are usable to gain additional energy anyway, why not choose ads as a method of F2P players to earn them? The revenue gained from ads may be small but it's still an additional revenue stream that could be tied to something that'd be given to non-paying F2P users anyway.
Not planned feature