Change of brake pads = change of rotors? Fact or myth?

[Lotus Man]

So I was told that with BMWs, once the pads wear out, the rotors are pretty much done, too. Is this true? Of the numerous vehicles I've owned, I've never had one that worked this way. For once, I'd like to get a least a couple of brake pad changes in on the same set of rotors, before shelling out big bucks for complete pads/rotors replacement with every brake service!

My three theories on why rotors have to be changed before getting too thin:

1. The walls wear down so much that they risk exposing the ventilated channel veins (inside the walls). Once exposed, the pads will catch on these exposed veins and shatter the entire rotor. But seriously, that wall is so thick, that it would take a lifetime of pad changes before even disappearing to expose the center ventilated channels.

2. Too thin of rotor walls present too much clearance for pads to make contact. Brand new pads at full caliper piston extension can only reach so far towards the rotors, so having the rotors too thin would imply the pads and brake pistons reaching their maximum distance of travel, and still not make contact with the rotors. But seriously, are the tolerances on the piston/pad travel that small that slight change in rotor thickness would affect grip?

3. Brake piston/pad barely makes contact the thin rotor. But they do the job in stopping the vehicle. But the brake piston is extruded past normal service distance from the caliper housing, just to assist the pad in making contact with the rotor, that the surface around the piston (outside the o-ring seal) is constantly being exposed to environmental elements/brake dust, resulting in premature wear and tear. But am I thinking too much?

In essence, I'm starting to sense a conspiracy theory here -- when I just want to replace brake pads in my X5, I'm always told that the rotors pretty much have worn down to minimum thickness requirements and that I need to replace them, too (despite them being brand new when installed with the same set of pads!). Somebody please verify this. Thank you!