Well, to cover my ass, here is my understanding of that tiny screw... It has no bearing on the structural integrity of the braking system. As mentioned before, from how I've seen it on my car, it's there mearly to hold the disk on the hub as you change a tire or what not. Assuming you can put on the new rotor without any hassel, and the wheel can still be secured, I don't see a problem. But, Nick, I'm no mechanic. I'm not pretending to be one. I've done my own brakes, seen that shitty screw. I can look at the lugs that I put through the wheel and rotor and having five of them, can conclude that due to the diameter of each one, it can be assumed that they are the effective connection (only real connection) between the hub, rotor, and wheel.
The lifespan of the hub, rotor, wheel connection would be as long as the five lugs will last. Any reasonable mileage wouldn't do any harm. But mind you, that is assuming that the rotor is flush to the hub.
Buddy, goodluck, be careful, just incase....