think you've asked the wrong question: why should fighters have to depend on casters to get them into the air? The answer is, they don't. Fighters can use mechanical means (airships and the like), flying mounts, and can use grafts. They use magic weapons and magic armor so what's the grief here?
I think having a flying mount would be preferable to taking this feat. You can't disarm a mount. You never want to be disarmed when using this feat. The Improved Disarmed feat just begs to be used on someone with Whirlybird feat.
Another drawback to combat with whirlybird is what happens when you make that critical fumble. Dropping your weapon is very bad if you are flying with Whirlybird feat. And good luck regaining your weapon. You are not falling at the same speed as your weapon due to air resistance. For all this talk of how thick D&D air is (see earlier posts) this means you will have to make a DC 25 Dexterity check (no skill seems appropriate to this) to regain your weapon while both you and it are falling. If you had the Snatch Arrow feat, I'd give you a +4 circumstance bonus for the feat. I'd also add a -2 penalty for every 10 feet away from you your weapon is. In 3 dimensions this can be tricky. It could be 5 feet to your left but 30 feet above you.
[Edit] As for dragons flapping their wings to maintain flight--wings allow you to grab up currents. That's why gliders works. Dragons can attack while gliding. I personally don't allow wing buffets while flying since I think that's improbable but [YMMVsup]1[/sup] However, if you are twirling a weapon fast enough to make flight (which rather boggles the mind to start and looks ridiculous), then how do you expect to maintain flight if you stop. Being hit with a simple hold person spell is decidedly more dangerous.
1 Your Mileage May Vary.