The cost, in terms of last-minute concessions to bring over the Bundesrat's supposedly neutral block, is not much more than the extra DM6 billion in tax revenue likely to come from faster-than- expected growth this year.
Based on brief segments of footage shown off, it seems like this ability should be used fairly often, as a well-timed block will mitigate damage and stagger an enemy, giving you an opportunity to land a few extra hits without fear of retaliation.