Let's suppose this is implemented. Who's to say one arrangement needs a simplified version and another one doesn't? If the player doesn't have the necessary skill to play a certain arrangement, then it's up to the player to simplify it to match their personal skills, and maybe progressively adding what has been removed.
I'm totally with Olimar on this one, a song could be simplified almost to infinity, it would be complicate things way too much. As for the duets argument, that's another story that has nothing to do with this. Duets stand as something independent, not as an extension of solos. Not to mention that I suppose what you guys are saying is that duets should also have easy versions? What really matters in the end is actual playability of what's written, nothing else.
Why shouldn't be this allowed? Well aside from what's above, why shouldn't remixes be allowed? Why shouldn't orchestral arrangements be allowed? Why shouldn't clarinet-only arrangements be allowed? Consistency.
Edit: Ok, you could say, "But virtuoso arrangements are technically playable, so they should be allowed following your reasoning!". A virtuoso arrangement is an expansion of a song, almost a remix, where the arranger uses the original song as a model to create something impressive, and, well, virtuoso. It would be the same as simplifying a song, but inverted, making it more difficult instead.
That way the other arguments apply too, who's to say this arrangement is too difficult or not difficult enough? Both extremes are too vague and differ a lot depending on the person so as to mark a "Standard-easy" and "Standard-difficult". "Standard-difficulty", on the other hand, is determined by the original song, therefore making it plausible.