I guess I am going to be shot down here but I really don't see the problem with cover sills. I accept that they are not original but if the inner sill has been repaired or replaced professionally by that I mean all the corrosion removed and then spayed with weld through zinc primer and then cover sills fitted, and the cavity filled with a wax type corrosion protector, then I really don't see the problem. But at the end of the day its up to the individual and there preference and as long as they are not fitted to hide anything nasty.. maybe that's is the only problem, if you buy a car with them how do you know... See I can even argue with myself sometimes !
Ignoring the fact that they trap moisture witch has nothing to do with being left out in the rain. As you will always get condensation inside a steel box section, hence why the sills are vented. And oversills don't offer the same structural strength as proper sills do. I once drove a mini with oversills fitted both sides and before a pair of popper sills were weed on, when pushed (hard not to with so many roundabouts and mg metro 1275) the doors used to creek on their hinges as the shell flexed.
That's interesting. My Mini has oversills, and I intend to sort them this summer.
However when I go into my garage, I have to go up a small ramp. If I open the door whilst the car is halfway up it (front wheel up the slope, back wheel on flat ground) then the door latch rubs on the striker plate. If the car is level that doesn't happen. Sounds like the shell on mine could be flexing as well.
Shells flex if you subject them to enough movement from new. I had a 309 GTI brand new in 1989 and you could not even close the back doors if you parked the front on a decent curb.