Because the radial speed is higher at the disc periphery they tend to wear more there than closer to the centre. This creates the wear lip on the circumference and leads to both discs and pads becoming angled over time. If you fit new pads to old discs the pads will only contact the discs at the inner edges until they wear in (if they ever do). This leads to all sorts of problems as the caliper piston tries to tilt the pad to match the disc surface and creates bending forces in the caliper, the pad, piston and the disc itself.
I think the judder problem starts if the inner and outer face of the discs are not the same shape so you get different contact forces from the inner and outer pads. I always skim old discs before fitting new pads and have never had a judder problem but not everyone has the luxury of owning their own lathe.
As soon as you mount an old disc on the lathe and start to skim it the difference in thickness at the outer and inner edges becomes very apparent. Generally I have to skim about 0.5mm off the whole face before the outer edge cleans up fully. Done on both sides this means about 1mm metal removal to get perfect parallel disc faces. You can do this once and then after using a second set of pads the discs are usually finished and well below the recommended wear limit which I always ignore as it's very overspecced. I have no problem using vented discs to about 2mm below the wear limit but if you push it much more than that they can crack.
On my own Focus the OE pads were shot at about 35,000 miles and after skimming the discs those were about down to the supposed wear limit. They're still working fine at 50k miles on the second set of pads and at 60k I'll replace the lot. The rear pads lasted almost 50k miles and I've recently replaced those and skimmed the discs. At 100k or a bit under both of those will be due for a change but that's many years away at my annual mileage.