Rito: Stormraider's Surge is inherently bad unless the numbers are off the chart

The reason that no one runs Stormraiders Surge isn't because its base stats aren't good enough. It's because it's a win-more. You actually have to be in a position to do that much damage to use it, but in the majority of games you're going to need help to do that much damage, so you take some other spell, one that actually helps you deal damage, instead of one where you hope you will. Yeah, most assassins will deal that much damage by the time they get about 2 items or so with a full combo, but you can't do that much damage before your first back and you can't do anything with the movespeed that early anyway. However, if you take a spell that helps you deal damage, you can actually kill your lane opponent before the first back or sometime before 2 items. If you take a damage keystone and snowball, you do more damage later in the game anyway. Besides, any assassin (by design, not counting like Assassin Jarvan or anything) worth their salt has a way to get out or stick to their target anyway.

The only time you would take Stormraiders is... well... never really. Why take a win-more keystone?