This falls into Brassman's issue with traditional hate pieces. The opponent plays a Sphere or has a Force in hand, you play this and it's essentially a 1-for-1 trade. It actually doesn't stop Mental Misstep as worded. The Phyrexian Mana is part of the mana cost, which is why you can cast if off of Snapcaster Mage but you can't cast a Force of Will. Ravager Shops isn't overly dependent on Spheres but uses them as a tempo boost and you are providing them that by casting the spell. In most cases, I'd rather have a removal spell as that's a 1-for-1 trade that I get to choose what to target.
I think the card would see play in various Storm decks as it essentially allows them to circumvent the most commonly played hate (Force, MBT, Spheres), consolidating the slots dedicated to Hurkyl's Recall and Defense Grid. I don't see that as an improving the Vintage format though. You are making it harder to interact with one of the more broken decks in the format. In essence, it's an example of my concern in the other thread of a hate card unlikely to have its intended purpose and inadvertently leading to worse gameplay.
edit: @Brass-Man jinx...dammit. You win that one.