In-game, dominating someone should allow dropping a weapon -perhaps throwing it a bit (though for most weapons, that wont be far). As a player, I'd expect my DM to do exactly that if that's what works best. If I just killed that mind-flayer's brood, I don't expect to be coddled by it - that's ridiculous and absolutely not acceptable in any way - I'd much, much rather accept an occasional minor imbalance than a broken story. It's about the story, not the balance.
Having said that, it's perfectly reasonable to adapt the rules, since there is something fishy going on. It's not reasonable, after all, for player's and monsters to react so differently to the loss of a weapon/implement. I'd say it's only natural that a monster has an unarmed attack that's no more than +3 worse than it's normal attack and perhaps a slightly lower damage die. But, conversely, if a "monster" human fighter takes only that penalty, well, then a "hero" human fighter shouldn't be dramatically different.
For that matter, picking up a weapon is just a minor action - is it not? So usually, throwing away the weapon is just a waste of time. Additionally, since PC's do more damage and have less hitpoints that monsters anyhow, the "charge ally and provoke a bunch of OA's" strategy works much better against PC's than for PC's.
It's be an odd situation in which throwing away a weapon - if fairly adjudicated - would be better than that. A reasonable barbarian, for instance, can drop an ally in one crit, if he's charging...