Leatherhead
Possibly a Idiot.
You're gonna have to work very, very hard to prove that statement.
I can claim with confidence that a football team works better with 10 receiver, one QB and no tackle, that doesn't make it true.
No, I'm not actually, the reasoning is ludicrously simple.
Not getting hit is better than getting hit. One of the best ways to avoid getting hit is to have as much distance between you and the guy who is trying to kill you as possible. In an MMORPG (forgive me for invoking this), this concept is most commonly known as "kiting." Another similar concept, the "stun-lock" hinges on using various forms of control to beat the enemy senseless while denying their ability to act at all. Nether method requires any kind of tank.
The ranged group works by keeping enemies out of melee range, using the mobility of strikers to play keep away, and the condition inducing powers of controllers to lock down those enemies with ranged attacks or superior mobility of their own. Naturally this isn't an infallible strategy, sometimes an enemy is going to get by or ambush the party, but neither is using a defender when the enemy starts flying or has ranged units of their own.
Defenders require groups to support their style of gameplay, which in turn makes people believe that everyone else is supposed to fit in around a defender, causing them to select powers and adapt strategies based on having a defender in the party. When everyone is fitted around a defender it does seem like the defender is a necessity, however it is quite possible to get along without a defender entirely, if you remove them in the beginning and have everyone work from there.
When your party has a defender in it, the defender has to get into the "High Danger Zone" right next to most monsters and NPCs. This is fine when you have other melee melee strikers to take a few hits and use combat advantage to ramp up all the damage for those people in melee.
In contrast, the defender in a ranged group isn't granting or receiving combat advantage as much, lowering the defenders damage, and even sometimes getting in the way of Area Powers, lowering the damage or increasing the risk of using the powers for the other classes. More importantly, the defender in a ranged group is forced to use more healing surges than their counterpart in a more melee-focused group, which cuts down on the amount of combats a party can do before it must rest. And for a minor annoyance, the ranged group is now tethered around the defender so that the defender can do their job, cutting down on their mobility a bit, but most encounters have some kind of range limit anyway, so this only applies in a few situations.
For an anecdote on the extreme side, I can recall when a Fighter in one of my games managed to place himself in a doorway between the monsters and the rest of the PCs. The Fighter ended up doing his job a little too well, having to spend over 10 healing surges that fight.
And to be quite frank, combat isn't like football, combat doesn't have as many limitations.