it can make it much easier to do on-demand migrations: in massive online apps, a large percentage of data is essentially 'dead' - it's unlikely anyone will ever access it again. and massive online apps (call it 'web-scale' if you will ;) can accumulate a lot of data. the systems they're discussing make it easier to migrate data as needed instead of at once (which sometimes isn't even possible if the infrastructure is really being pounded).