Yeah, I think you assume too much about how the game is SUPPOSED to be played by DMs and players as opposed to how it really should be handled by intelligent or outnumbering foes.
As one example:
http://www.enworld.org/forum/living-eberron/311263-adventure-rhapsody-part-2-judge-renau1g-9.html
This was an N+3 encounter. The Warden got bloodied, but the Wizard and the Invoker got knocked unconscious once each. The animal companion (who are significantly lower in hit points than PCs) got killed 3 times. The Warden tried to keep foes away from the other PCs as did the Shaman|Druid with summoned and conjured creatures, but the problem comes in when Wizards and other squishy PCs just don't have room to run away, or to get a cover bonus, and/or one or more foes concentrate on them.
Sure, in small rooms with an equal number of foes max, a Defender can often hold off enough foes so that the other PCs can handle the rest. And, Defenders also tend to have powers to bring some foes to them or to penalize a foe for attacking someone else other than the Defender. Defenders work especially well against solos.
But, the concept that there is balance here between the classes only works if the DM is willing to play his NPCs in a certain (rather stupid metagaming) ways, both in NPC decision making and in dungeon design. A reasonably intelligent NPC with a melee attack should rarely try to go toe to toe with a heavily armored PC. Does it make sense to do so? But I sometimes see some DMs have even mobile NPCs go up and do just that encounter after encounter.
In 4E since they changed the charge rules, it makes sense for most semi-intelligent melee NPCs (and even ranged NPCs) to ignore anyone who looks hard to take out, move around them and charge/focus on the PCs that look easiest to take out. Dumb NPCs? Sure. They should often target the closest PC.
Extremely intelligent NPCs should shout directions out to allies to attack certain dangerous and/or lightly armored PCs (which if you go read the attached link, that is what my leader type NPCs do a lot). As an example, once a PC heals other PCs, that PC should become a prime target candidate. If a PC (really, not just minorly) locks down a few NPCs, that PC should become a prime target candidate. If a PC starts doing a lot of damage, that PC should become a prime target candidate. But until the NPCs can tell which PCs are doing what, they should focus on the softest looking PCs. I cannot think of many reasons that a Defender PC should become a prime target, but I can think of a lot of ways for many NPCs to get around Defenders.
And, my analysis above only took into account damage. Another major problem with low defenses for squishy PCs is the rider effects that can occur with a hit. The first level Wizard getting hit 60% of the time compared to the Fighter at 35% of the time means that the Wizard has a higher chance of getting some effect on him. The concept that Wizards shouldn't use defensive feats and should be more cannon and less glass won't work in a game where the DM challenges all of his players.
So yes, there is nothing wrong with having squishy PCs as long as the squishy PCs get over compensated such that they are taking out (either with damage or nearly total control) more NPCs than the non-squishy PCs. I just don't see that with 4E Wizards unless that Wizard uses a Daily attack power and they only get at most 2 of those per day until level 10. Dazing or slowing or knocking down or even moving an NPC around usually just doesn't cut it with respect to power and/or versatility in comparison to how defensively weak Wizards are designed to be.