I cited Throes because its the primary self-repeatable Cascade card, which is what your card does with dash. Ingenuity Engine is similar to Throes, but you have to sac an artifact instead of exiling a land card from the graveyard, so you are still spending a card. Phoenix Requires you to have another Cascade card to get it back and Sphinx can be cast, at most, every three turns. I'm assuming you meant Etherium-Horn Sorcerer, not Etherium Sculpter, but there you still have to pay extra mana to set up another usage of cascade, meaning you pay 9 to cascade off a 6 mana spell.
Summer Rain combines the cascade and recursion into the same cost so you can dash it turn over turn with no interruption and not running out of a resource to keep doing it. That ability is strong inherently, without also being able to additionally hit your opponent for three each time you do. Yes, its a weak creature and decks running burn or counterspells can answer it easily, but decks that rely on creature combat for interaction or board wipes/sorcery speed removal cannot interact with the cascade so making it more easily blockable is a reasonable way to reduce the card advantage this is granting.
Yes, I did mean Sorceror, thank you for being so charitable.
I'm sorry Rowan, I just don't agree. How many times a game do you think you're going to cast a 6-7 mana-cost spell that leaves you with shields down and not win?