It is like the complaint that it isn't fair that a fighter focussed on brawling isn't any good against flying opponents. Tough. If you want to focus a character so much on one theme that you can't deal with the few foes which are not playing in your space, then you have to accept that you either need friends who can cover your back or you need to find coping mechanisms for those circumstances.
This sets off my "Different Playstyles!" alarms.
Some games -- especially on the more sim/sandboxy end of the spectrum -- hold that this sort of blatant categorical ineffectiveness is part of how you choose different missions, or make different parties, or use different tools. Part of the play experience is using the world to your greatest advantage. There's no problem not being able to take out flying enemies, because you're never without the option to
not engage those enemies.
Other games -- more on the story/narrative end of the spectrum -- hold that this is a problem, because the play experience is more about your heroes and the big villain and how they fight and who they are. There's a HUGE problem if you can't take out a category of enemy, because you're expected to be the hero, and if you're not, it's not fun for you.
4e is much more heavily weighted to the story/narrative side of things in this situation. I can't say I blame them. If they expect you to stay with one character for two and a half years, over 300 encounters, they're going to want to make
that character an effective contributor in nearly all of them. Sure, certain encounters will skew certain ways -- defenders and strikers shine during solo battles, controllers are less useful without minions, leaders suffer from long encounters with high-damage enemies -- but for 295 of them, you should be doing something to contribute to success.
If those 300 encounters were composed largely of undead and shadow enemies, or demons and devils and fire elementals, or rocs and pegasi and flying wizards, you'd have a harder time feeling like you are the hero, and, unless the game was deliberately sandboxy and you could choose to go elsewhere (e.g.: the DM has planned that the big threats in the campaign are flying undead fire elementals!), it's going to suck.
As WotC, I would be interested more in the latter half, since there is more at stake there -- a bigger penalty for failure. The worst that could happen is that some sensitive DMs are upset when their waves of fire elementals fail to make the player of the fire mage cry in futility. Which sounds just fine to me.
