If the mount gives 8000hp at lv 1 for example (the dragon mount). That means every hero you will use will have an additional 8000 hp...then lv 2, 16000, then 24000. This adds up to an amazing boost. Of all the defense stats, somehow I believe that HP is the best one. For every day that you feed a lousy mount you are losing feed at bigger mount...meaning if at all possible, get the best mount you can as early as possible. Otherwise you will end up spending 1000 gems to transfer....else lose a month of work. Runes are extremely important and can be maxed with hard work and without coining, but their cost essentially is the same throughout the game, while mounts are not. Additionally the really good mounts get bought fast, so buy the best one the fastest
Griffin