Skyscale, Griffon, and Beetle were the mobility and terrain kings. The siege turtle does not give an advantage dmg or terrain wise, but it does provide an advantage for you in new situations you might not be familiar with. Let me explain. If you are in unfamiliar territory or at a boss fight, You might not know what to expect, especially in some of the more deadly areas of Tyria. You have a much greater amount of HP on the turtle mount so it provides a safer way to explore and can be used to learn the mechanics of a boss or area so you don't get downed quite as fast. This will not apply as much to seasoned players but even for vets, its kind of nice to chill in one for a while. I can also see it being useful for times when there is unavoidable damage.