I'd say it isn't impossible to break an arm standing (as proven by Aoki, possibly one of the best MMA'ers today). But it is a lot harder to get the leverage to do so when your opponent is free to move. Your options of pinning your opponent down and limiting his escapes are severely diminished when you are standing.
Plus the same really goes for this technique as for any other technique that you wouldn't do on your training partners (unless you're a douchebag), it will never get trained and thus very seldomly get used in any kind of real situation (let alone made to work). You fight how you train, and straight out breaking someones arm isn't something that you can train (and yes it's quite different from training joint locks i'd say, since there's really no intermediate stage where your opponent can tap since he feels the pressure/pain).
Free thinkers are dangerous!