Right, machining keyways into the crankshaft, camshafts, damper, and their respective sprockets is not free. These keyways are typically only a short radius away from centerline. Therefore, high accuracy (tight tolerances) are necessary in order to have good timing accuracy. The keyway also is a significant source of stress concentration which can lead to fatigue failure.
Removing the keyways makes it easier to have high timing accuracy at lower cost or even better accuracy for maybe the same cost. I don't know the real cost to benefit analysis. On the I4 Duratecs, the crankshaft timing pin tool contacts on the outside of the crank counterweight which is a much farther radius out from centerline than the outside diameter of the crank snout. The slots across the backs of the cams are carryover from the old Zetec engines, so that isn't anything new. Even uses the same timing tool. It's about the best of an alignment method you can get on the camshaft without having some part have a large radius which takes up space. The full length slot (2x radius length) is better than a keyway (1x radius length) assuming the radii are equal.
Sure, there is more risk of the timing slipping. In fact, they did have issues with the timing slipping at the crank of some of the first I4 prototypes. That is why the very thin friction shims were added to both sides of the crankshaft sprocket. These provide a 2 or more times multiplier of the coefficient of friction between the surfaces. If you do any bolted joint design, you will know how much of a huge help this would be in keeping surfaces in shear from slipping. If you google ESK friction shim, you will find the Ford I4 engine crank and crank sprocket setup. Ford is not the only OE using friction shims in the powertrain.