I was never suggesting that PCs attack one another if one is flying. That's an inordinately bad idea. However, as DM, I think clever monsters should use clever techniques (if you have ever heard of Tucker's kobolds this should come as no surprise). That said, I do NOT advocating killing PCs for the DMs pleasure either. That is a clear sign of either immaturity or lack of experience. D&D is a cooperative game. Putting PCs in extended peril is far more entertaining for all involved.
That said, a lot can happen to a falling PC before he hits the ground. My goal as DM is not to kill the PCs but to keep the Players engaged for as long as possible.
Aerial combat can always attract wandering flying monsters or even a new NPC. Not all NPCs who rescue you are Good and rescue can come at a hefty price. Nothing like making a PC owe a favor makes for good drama. As a final teaser, I can keep the falling weapon just out reach until the 11th hour or until the other PCs can make up their own heroic rescue.
The goal is not to let the PC hit the ground and die, the goal is to keep the tension going for as long as possible. In my not so humble opinion, that's the difference between a good DM and a lousy one. D&D is a cooperative game not DM vs. PCs.
Anyhow getting back to the feat. To make this more like the fly spell, I've made a few changes. Note the duration of the fly spell is only one minute per caster level. This lasts longer and can be used outside of combat now. I consider flying this way akin to hustle movement rather than walking movement hence the fatigue.
Whirlybird (Fighter) Feat
You can swing your ax so fast you fly like a bird
Prerequisites: Two Weapon Fighting, Weapon Focus (Orc double ax), Str 20, Dex 15
Benefit: When using the full attack action with an orc double ax, you sacrifice your highest level attack in order to fly 10 ft. with good maneuverability. You may attack foes with your lesser attacks (including off-hand attack), and then fly, but only if you have not used your highest attack first.
Once you are flying, you can maintain the flight as a full-round action. You can fly for a maximum of one hour before becoming fatigued and must rest for 8 hours before flying again.
Normal: You cannot fly
Special: A fighter can select this feat as one of her fighter bonus feats.
How is this?