Mate, the max difference between having a weapon and not isn't +6, it's +9. +11 if you're pushing the envelope.
This is assuming you don't carry a backup weapon, if at an epic level and a PC doesn't do that, well, tough there. Like I said, classical - medieval - modern era, anyone that's going to be out and about have back up weaponry. Hoplites have their two javelins (and sometime shortsword/daggers), knights have their main-weapon + long narrow daggers (for piercing between gaps on armoured units) or shortsword, modern soldiers have their weapon + sidearm, even cops have sidearm + nightstick. In the fantasy genre, you have halflings carrying slings + daggers, kenders with hoopaks + dagger, beastmen that uses sword and then their claws, etc.... The only excuse not to carry a backup (even a mundane) is if the DM/Campaign specifically tell you not to, otherwise, it's the PC's own fault (be it arrogance or naivity). The book doesn't assume how much weapon you are carrying, the treasure part only assumes 1 "primary" weapon.
In the case of improvised stuff, they don't cover as much as that would limit the imagination process (I hope that's the intention anyways). If the DM is going to be using disarm, s/he should let the group know ahead of time if the situation can end up in death. But.... having more than 1 weapon is normal, having only 1 seems... like something one is playing on a field where everything goes their way. Fighters carry range weapons since they can be immobilized and out of range of melee, rangers have both range and melee weapons, or at the very least 2 melee weapons, the list goes on (except for casters).
What's more, you can't use fluff to justify dodgy mechanics!
If you break the game, then say "that's ok, you can figure something else to unbreak it on the fly", then you're being outright silly.
This is D&D. Improvised actions should always be, at best, slightly less powerful than an at-will.
Is disarming in an ettin's thought processes?!?!
WTF?!
How can it be in an ettin's fluff box, when disarming does not exist as a mechanic?
If it can be done, and experience shows it's brutally effective, then any creature capable of learning would learn to do it!
A 45% swing in hit probability due to one action is outright retarded .. if you have a 50% probability of hitting (as a moderately effective character should), that leaves you with 20s. Not to mention doing <15 damage on a 7[W] attack even if you do hit.
The mechanics are only as dodgy as your DM makes it since it's "does not exist". The design process for anything not in the book requires contingency vs said action (refer to the DMG). Make the disarm process more painful on failure, etc... like you said, improvise shouldn't be better than at wills, so disarm would not be better than any at-wills in the first place, there could be major drawbacks (if prone is for failing an acrobatic stunt, there could be many worse thing it can end up with on a failed disarm attempt, OA is just the tip of the iceberg). It's heavily dependent on the campaign itself and how your DM fits it in the game. Make it so that the target must be bloodied AND require combat advantage will be one way to restrict its use amount MANY other methods.
Monsters have thinking process too, otherwise why would there be tactic paragraphs? Would a scathe beast knows how to disarm a PC, or would it just ram into it and try to grab it as its nature?
Fluff not discerning enough? Let's take it apart should we?
ETTINS ARE RAVENOUS TWO-HEADED GIANTS that prowl wild
borderlands, forested mountains, and dark caves.
Ettin Marauder Tactics
An ettin marauder engages foes in melee combat, spending an
action point if necessary to reach a lightly armored adversary.
It uses swat against an enemy that tries to flank it.
From the fluff alone, there is NO indication that Ettins are tactical thinkers (more on the savage side). There is nothing in their about "trying to gain advantages". So, tell me, why would an Ettin even attempt to disarm someone when they can squash them like bugs? The fluff, believe it or not, tells a lot about the thinking process of the monsters and how they should act-play out. Have a dragonborn that knows his stuff? Disarm sounds a lot more reasonable. Disarm is maybe effective, but would it be the creature's nature to do it? Using disarm should be used when it makes sense. The way you make it sound as though if one creature knows, all the other creatures should know and use it on the PC. I can not imagine an ochre jelly or many other creature even attempt to disarm. If they do, it should be the exception, not the norm.
Moving on, you are referring to straight damage as though that's the ONLY thing you can do to help your allies, you don't HAVE to deal damage to be helpful to your allies, you make it sound as though losing one weapon will be the end of the PC's life (funny how I had an encounter where the 4 PCs were caught and sent to a little dungeon and lost their magic items temporarily and still whip the enemies' ass on normal encounter difficulty). The battle might take longer, but it isn't just "power A to target B, add/minus numbers", creative thinking were done and the fight was fought effectively, using the layout of the map as well as improvised weapons. Improvising on the fly IS encourage (and if something breaks, unbreak it on the fly IS reasonable -human makes mistakes, DMs included- as long as it keeps the story intact, or it maintains the fun level, or many other reasons). If improvisation isn't to your taste, or thinking on your feet is too time consuming for your group since they have rule lawyers in the closet for everything, houserule that only actions in the book can be used, or just use another system all together. Disarmed != useless, like I said, it's ONLY as useless as you (and to add, your DM) make it.
As for creature learning, how about the side effect of attempted disarm? It haven't come up in any game I've run or played since even doing disarm isn't that major a factor since the failure rate is simply too high (vs reflex/fortitude, whichever is higher and the penalty to hit AND required conditions). It's brutally effective, sure, if it lands in the first place. But on the opposite end, the PCs will probably give it such a whooping right after a disarm failure that the other monsters learn NOT to try and disarm unless under extremely favourable position (IE. NPCs-Mobs are clearly winning and they want to capture live preys).
The system does not support disarms.
There are no maneuvers to counter it, there's no equipment which increases resistance to it, the treasure you get assumes you're not going to need more than 1 or 2 weapons, and half the powers in the book assume you have a weapon along with the corresponding bonuses available at all times.
When a system does not support a mechanic, and the mechanic disrupts the maths to an extent where the declaration of "paladins suck because they always get disarmed!" it breaks the game.
That's when the DM uses his power via the "Golden Rule" (DM makes the final decision). If it disrupts the game, the DM should be actively limit - discourage disarming via limitation, requirements, etc... The system supports ANYTHING as long as there are counters to it. No maneuvers or too lazy to think one up (disarm is a thought out action)? No resistance to it? I guess reflex and fortitude defenses(or whichever defense the DM decides to use) can't be increased no matter what X magic item +Y the PC wears. Want specific items to reduce the already low disarm success rates? Think one up! The disarm was thought up by your DM, there should be no reason why s/he can't modify an existing item to suit the "new" mechanic.
If all else fails, as other's have said, don't allow disarm unless it is in the statblock (and any improvised/"does not exist" actions like swashbucketing, PCs using traps, etc...) in the first place, there should be enough action inside the box to satisfy most players that don't want to think of/worry about these ideas.
Back to the original topic, does 4E have disarm? If the group and the DM allows it, it certainly does. If said group and DM disallow it, then it doesn't.