Realism: Classes with the Evasion ability must move out of the area of the blast if they make their save. I can't see a person staying in the blast, yet avoiding all damage.
Characters with mounts with Evasion or Improved Evasion gain the benefits of either while mounted. If the mount successfully saves and evades all damage from the blast, so does the rider. I know it gives the Paladin a little something extra, but it fits better with how I see Evasion working. And I like the imagery of a Paladin's mount avoiding dragon breath and wizard spells while the Paladin fights courageously to defeat the evil. This rule alteration was a mix between Realism and Flavor.