Of all characters that should be getting a new move... Magneto is not one of them. He is a complete character in UMVC3 and with a few modifications (like take out the Repulsion exploit) he would be a perfectly legit character that requires effort to utilize him properly but when you do you are rewarded. He already has more than enough tools to compete.
It's like giving Dante a new move. Just a case of rich getting richer, that's all.
See, I have two problems with your line of thinking:
1) You are making a false equivalency between a character gaining new moves and things like balance and competitive viability, which are not necessarily related. Adding a move does not necessarily result in a character becoming unbalanced and/or stronger competitively. (It does add additional variables that need to be taken into consideration when balancing though, which theoretically would make it more difficult to balance characters and would yield a greater probability for negative balance issues to result, so I can see why you would say that.)
2) This is a game where a lot of the interest in particulars characters derives from how accurately they are adapted from their source material. When playing UMvC3, many of the characters play very similarly to the way you would imagine them playing if you had knowledge of their representation in their source material. In my opinion, adding to a character to more faithfully represent source material is a good thing. Does UMvC3 Dante really need that many moves? Absolutely not - he could be just as well balanced if you halved the amount of moves he has, but having all those moves gives you a greater sense of playing a DMC3 Dante ripped straight from that game and placed into MvC3. Additionally, with "legacy characters" (e.g. Magneto, Spider-Man) their moveset was constrained previously to the imagination of the previous design teams and the ability to implement it into the previous iterations in the Vs. series (which could've been constrained by the game engine, system memory, control layout, etc.).
Finishing my thoughts with looping back around to Magneto's new move: though attraction/repulsion has shown to be largely situational and unused in high-level play, I'll be damned if it doesn't make UMvC3 Magneto play a little bit more how you would expect a comic book Magneto to play.