The seems to be a disconnect between people wanting Disarm for narrative purposes and those who want more mechanical options - or shortcuts - for killing stuff.
I think the disconnect is between people wanting a mechanic to re-introduce Disarm into the game and recognizing how powerful such a manuever is, especially in 4E.
Really, what other power in the game can deny a character all of their own weapon based powers? What other
action could do this, since it's not even a power, but something anyone could do?
Being disarmed, in game terms, is a penalty with no save. If the character cannot recover their weapon, it can effectively be a Save or Die effect (the save in this case the initial attempt to resist the disarm.)
Remedies suggested include simply requiring the Disarmed character to spend an action to retrieve their weapon. But in 4E, it's far too easy to separate a character from a dropped weapon, so this isn't much of a solution. A PC could potentially lose their weapon permanently, much like the 3.x sunder action.
Another suggested remedy is the ability to pull out a backup weapon. But in those cases where a target doesn't have a backup weapon, like most great-axe weilding orcs? We just shrug our shoulders and say "Oh, well?"
Even if a chartacter pulls out a second weapon, especially a PC, it's likely they will experience a penalty compared to using their primary weapon. This will especially be true for wielders of heavier weapons, who are unlikely to carry an equivalant weapon for backup. A character is likely losing his original weapon's magic bonus, higher base damage and effects/damage of powers reliant on weapon type. So multiple penalties lasting the entire encounter with no save.
And of course, a backup weapon is no guarantee a Disarm is reduced to just this set of penalties, as the backup weapon can also be disarmed, possibly in the same round as one faces multiple opponents.
Given 4E's higher HP and generally higher number of rounds per combat, Disarm becomes a much more effective way to take an opponent out. More effective than most powers.
And, given 4E's focus on balance, Disarm as a mechanical option is not balanced.
Cinematically, becoming disarmed usually signifies the end of a fight. Realistically, even if someone has a backup weapon, they're likely to get run through as soon as they reach for it. In those scenes where the fight continues after a disarm, it's usually a duel between two opponents. These scenes are rarely duplicated in a D&D game, as almost all battles are between groups of opponents, where being disarmed is even more likely to result in a surrender.
The suggestion of using the existing mechanic of letting the PC decide what happens when they reduce an enemy to zero hit points to declare a disarm is perfect from a narrative, cinematic perspective.