More options = more negatives? I see how it goes. If it's in 4e, it's got more positives than negatives. If it's not in 4e, it's got more negatives than positives. If it's coming out in a splatbook, it must have more positives than negatives.
Although you're obviously being sarcastic, you're actually more or less correct. The only real error is that you're expressing the relationship in reverse.
For the most part it's true that:
If it's got more positives than negatives, then it's in 4E.
If it's got more negatives than positives, then it's not in 4E.
One thing to remember when evaluating any new edition of any RPG, D&D included, is that the rules we, the players, finally see are the result of
years of work by people who design these games professionally and understand them much better than we ever will.
A disarm mechanic is an obvious addition to any combat-oriented RPG. In fact, 3rd Edition already had one right in the Player's Handbook. Therefore, the very fact that 4th Edition
doesn't include such a mechanic is extremely powerful evidence that one would detract from the game more than it would enhance it. It practically goes without saying that during the development of 4th Edition the designers had, at some point, included a disarm mechanic. Hell, they probably tried several different versions. But when the rules were finally published, no such mechanic was present. Why? Because they, the
experts on the intricacies of this system, had decided that the game was better off without one.
Moreover, a disarm mechanic is in no way special in this regard. There are doubtlessly dozens upon dozens of other ideas that were also tried and
rejected because what they brought to the game was outweighed by what they took away from it.
Long story short: If an idea doesn't appear in the game as printed, that's extremely good evidence that the game is better off without it. So unless you've got a better reason for supporting a disarm mechanic than "it's realistic", you should probably trust that the professionals made the right call.