Im FAR from an expert, but lets look at it in a scientific way, shall we?
Well, its all about what happens when the car is braking, right? Basically, the cinetic energy of the car is disipatted through friction (which generates heat n stuff, so disipates energy). Were talking about friction between the braking pads and the braking discs or - if the wheels are locked - friction between locked wheels and the road.
So, when it comes to locking or not locking the wheel it all comes in fact to which friction is better at dissipating energy. In other words, which friction is stronger. Keep in mind that with the ABS the friction between the brake pads and the discs is not constant.
Lets take the two extreme cases:
1. VERY DRY ROAD WITH GREAT FRICTION BETWEEN THE LOCKED WHEELS AND THE ROAD
In this conditions the friction between the locked wheels and the road might be better than the friction (taking into account those ABS interruptions) generated by the braking pads pressing on the discs. In this case the car that locks the wheels should stop faster. But it might be quite hard to find the wheels - road combination that will give that kind of friction. Plus the temperature of the tyres comes into play etc. At least the pads and discs are quite constant as far as friction goes, I guess. Or they should be, at least
2. VERY WET ROAD WITH NILL FRICTION BETWEEN THE LOCKED WHEELS AND THE ROAD
This is the exact opposite situation. Lock the wheels and youll have ZERO friction with the road, youll slip like a sledge and youll stop... sometime
Dont lock the wheels and the friction between the pads and the discs will stop you much faster.
So, at least in theory, its about which friction is stronger. Locked wheels and road or pads and discs? With the ABS youre not using the 1st category of friction when braking (well, just in theory, as the ABS allows small portions of skid anyway, as I know it). So, youre limited to the braking force of the pads and discs and - in certain conditions - the locked wheels and road combination could be better.
Of course, a lot of other factors come into play, but as I see it, this should be a rather good approximation.
Thats about it. Sorry for long post
-Da Newton