Take a look here: Muto vim errata edit , which itself links to more.
Erik Tyrell started off with a good point about Range. You cast a spell with Momentary MuVi bumping it from Voice to Sight, for example. This allows it to put an effect on a target far away. Why, now that the target is affected, does the spell suddenly bounce back, still active but with a shorter Range? And how does that even work? If the target gets closer the effect reactivates? Meanwhile, the core rules point out that once an effect is active, it will run on its own regardless of the Range the target moves to.
ExarKun makes a great logical point as well. MuVi cannot affect already-cast spells. So after the casting, the MuVi has no effect. If that's the case, the MuVi spell ending early shouldn't matter. So the original erratum created a paradox.
Besides, people arguing about how it messes with Wizard's Communion are in even worse shape with the old erratum. For example, with that you needed a Year-duration version of Wizard's Communion to work with AotH, which would make that version of Wizard's Communion itself a Ritual.