If you have more worn tyres on the front, then those tyres are more likely to lose grip in an emergency stop. In that case the ABS will have to step in and help regain control.
You could ask your local traffic Cop (if you can find one) whether he or she has been to more crashes where a front wheel drive car understeers on a bend or where they've oversteered or spun. I'll put money on it that they've attended far more where understeer has been a factor. Alternatively, a spin has occurred where there was initial understeer and the driver over-reacted with steering or brakes, which has itself caused the spin. Even in the speed-camera-crash type fly on the wall Cop programmes, when a car being pursued crashes, it normally does so because it understeers. Get the turn-in right and you don't have to adjust through the bend as steering adjustment simply destabilses the car.
As I said in my earlier comment on this thread, in 43 years I've never oversteered a car on the road. Oversteer and understeer both have the effect of reducing speed, but as a device to slow you down, they're both rubbish.
While not doubting the accuracy of the four tests quoted, one, Michelin is a French company (and one that used to be noted for the longevity of its tyres rather than their grip) and the other three are conducted by organisations in Germany, Austria and Switzerland. All four countries have two things that we in the UK don't get much of. Ice and snow. With that in mind, I think you have to consider that those tests will reflect the driving conditions in those countries. I only de-iced my car three times last winter.
Ok, I'll bite. I'm not disagreeing with you regarding the fact that most of the breaking effort is done by the front wheels.
As already said, car manufacturers set cars up to understeer when pushed. A look at the suspension geometry for a 'normal' car will confirm this. Figures I have in front of me are for the Fiat Ulysee, zero camber at the front and 1 degree negative camber at the rear. In short, increasing negative camber increases the grip when cornering.
Why do manufacturers set the car with understeer characteristics? It's because understeer is generally accepted as the less dangerous option. Understeer is fairly controlable and, more to the point, controlable without experience, as generally all that is needed is to reduce throttle input.
Skidding while braking is slightly different, but, if the car fails to stop, or runs wide while cornering, the result will mostly be a frontal collision, which the car's safety systems are heavily biased towards.
Oversteer will typically result in a side or rear collision, or a roll over (as in my accident).
Ask the traffic cop which of the crashes were most severe, those involving understeer or oversteer?
Essentially, the car manufacturers attempt to produce a car that will 'fail' in the safest way possible, by biasing rear grip when cornering.
The best answer is to change all four tyres at the same time, afterall, the manufacturer set the suspension with new tyres.
The second best is to put the best tyres on the rear, as we want to avoid oversteer at all costs.
A small amount of understeer or poor grip when braking will, hopefully, remind the driver that they need to get new tyres.
By fitting new tyres on the front, you are increasing the front grip versus the rear. If you manage to overcome the safety margin that is provided by the suspension geometry, you will have a car with sudden and severe oversteer, this generally requires new tyres on the front and very worn ones on the rear though.
There, that's the reasoning behind 'best tyres on the rear'.
As to the original question, I'm going with the worn front wishbone theory myself. It makes the front end more compliant and therefor makes the rear feel 'loose'.
Controlled oversteer on a racetrack and uncontrolled oversteer on the road are very different things.
