Im sure teams build their engines so that they can deliver 100% of their capabilities for 2 races. When you could use 1 engine every race they also didnt suddenly lose a couple of thenths after half the race, this is basically the same only instead of lasting 2 hours or so they now build them to last 4 (race engines arnt really build on distance but on time they have to run).
Well, I wouldn't be so sure. Actually all top 3 teams turned down the revs on their engines before the end of the first race. Actually Ferraris started saving their engines already before the first pit stop! And I'm not blaming them, one Ferrari engine did in fact blow up in Malaysia, I think it was one of them Toro Rossos'.
Basically what is happening is you want your engine to be as light as possible while still delivering maximum performance, hence reliability suffers.