What your seeing is normal, agreed.
No, it doesn't. The brake rotor holes filling with brake dust would not cause them to need to be replaced more often. It's just the nature of the cross drilling. Now, if you are on the track every weekend you will want to keep an eye on the rotors (especially the front) for hot spots, cracks around the drilled holes and pay attention to the brake pedal feel for warping. Under normal street driving conditions I would expect them to last a very long time.does this mean they would need to be replaced very often?