@MartinHarper:
Well, I'll be darned. Guess I've got a houserule, then.

Commence cries of "The game is broken if you have to fix it!", but I prefer the interpretation that views the whole thing as unfolding all at once instead of enemies who politely wait while you make multiple throws. Mechanically, the difference is almost nil. Though I suppose it will increase when we get Rogues up past early paragon and their level-appropriate gear only goes for the first attack. I'll see how that plays in practice... it might bug me less by then, too. I don't have a fundamental problem with the idea that an epic level piece of magical killing tool can be used for a single fistful-o-dagger attack.
And if you have an "orbizard party", you're asking for it. I've read the "same synergizes with same" theory here before, and I don't buy it. If you've got a party of 5 30 level orbizards, what's your move when you fight a loooot of little stuff? You can kill them with anything, but every turn they're alive they're going to be hitting you. And for my chaos hydras... well, I'd call the fight to get to kill Orcus with your one-shot big move "hard" encounter. A hard but level-appropriate encounter for a party of 5 30th level characters? Say hello to my friend, ten Chaos Hydras. That's boring but legal. If I were actually doing this, it would probably be "say hello to my friend, five Chaos Hydras plus a lot of smaller things." All the demons and undead to pick from? Yeah, it would not be a boring encounter. Actually, it probably wouldn't even be five Chaos Hydras. I'd keep them in there so there would some big guys you couldn't stun and some you could.
And it wouldn't be a cakewalk or a TPK for the party, either. That's the point. They'd win or lose based on their actions.
I'd kind of like to see this fight in action, actually. I wouldn't try to run it myself right now... haven't played epic yet. But I'm going to keep it in mind for a one-shot sometime, if nothing else.
I'm sure someone will say that this "stretching the game to the breaking point to patch a bad mechanic", or something, but it's really not. The game as written recognizes that not everyone's going to have the roles represented and it advises you to balance encounters differently for different party configurations. We can argue until the cows come home about whether such "soft mechanics" are "rules" as not, but it is how the game is written.
For in-game verisimilitude: there's a party of five wizards who have spent [30 levels worth of time] perfecting a magical technique that makes single large foes and they can slay an archdevil or demon lord or discorporate a deity with it very easily. All The Powers That Be adjust their personal security precautions to include things that are resistant to the most totally debilitating magics and to include large numbers of fierce creatures that can fly over zones and/or teleport and do loads of damage.
@Roy:
"Yes they do. The bow does the same or more damage, except now the MOBs have ranged. So they all get bows because any of them can. Then you lose because 4.Fail is incapable of being anything other than an MMO style stat contest, and the MOBs have bigger numbers. May the meaningless gods help you if they have wings."
First, I don't play with Mobile Objects in my game. I play with Non Player Characters. The key word is characters. They all have an internal logic and a life that exists beyond the encounter (at least stretching out into the past).
If one of them has a bow and can hit with it and do damage with it, it is because he or she or it is an archer. The game has no mechanic for monsters that says "this one can hit shit with a bow and that one can't", but if you're not after a video gamey experience, you shouldn't ignore matters of "in-game reality" like that. The dude who can cut shit up with a sword or halberd isn't trading for a bow, and the local evil overlord doesn't have unlimited skilled archers.
And if he did have loads of skilled archers, he'd still be smart to put some close fighters to put between them and the people they're putting arrows into.
Nothing stops me as DM from saying that everything down to level one minions can fly and has a bow and arrow that they can use in flight. The game mechanics will allow me to do this thing. But "things the DM can do" aren't game breakers. Rocks Fall; Everyone Dies. The DMG and the MM at the very least have encounter guidelines and examples of monsters and typical encounters that make for a fair challenge.
If I choose to ignore that and the game breaks, it's not a fault against the developers.
@PsychicRobot:
"The designers failed in their task because their system supports one playstyle, which is the Gauntlet style of play: charging into the middle of a group of enemies and spamming your best attacks. "
That's not how I play it. That
seems to me to be how you guys play it. The game comes off broken to you, it works just fine to me. We can say it's not a terribly flexible system if it works the way I play it and doesn't work the way you play it, but I can't really reconcile your statement that it only works when you play it like Gauntlet.
...
Really, I keep hearing different things... and I'm hearing them from different people, which might be part of it, but: there's no reason to not be orbizards, there's no reason to be anything but archery rangers, there's no reason to not be (one of the ubermelee builds)... Mongols break the game because they're unbeatable, Mongols with jetpacks break the game because they're unbeatable, the whole party should be melee, the whole party should be missile...
The only thing I can really take away from this is "Man, there are a lot of ways to play the game, and that makes it hard to come up with a strategy that wins in every situation."