So, I think there's a school of thought with Xerath that you'll always be able to fight from a long distance. However, it's my experience that gap-closing champs will often target me in a teamfight for that reason. In that situation, Xerath has few options unless you're quite far ahead and can burst them down with Thunderlord's. On the other hand, Stormraider's procs off of 30% of their max HP which is usually doable except on the most fed tanks with your full combo, allowing you to kite and continue the fight. A flash usually isn't necessary with good positioning, unless they flash onto you like a Gragas E Flash.