In this episode of Alpine Preparation, Rebecca Coles explains why you would choose to block lead when alpine climbing. She runs through the advantages and few tips which will ultimately help to speed you up when in the mountains.
Laying the ropes across your belay is all very pretty, but not something you'd do if using an auto lock device, you'd just let them fall in a pile on the ledge. If block leading you'd just rapidly re-pile it on the ground(back coil), whilst the other takes a few bits of gear off the 2nds harness and off you go. The alps is about speed and simplicity, much of the time you don't trust the belays enough to direct belay, you might semi direct, or even just old school body belay a 2nd up straightforward terrain, it's fast.