True. Marchisio exploded when Vucinic (especially in 4-3-3) operated with him on the left side and made those passes you're talking about. Claudio is really good with those penetrating runs. Excellent, actually. Llorente, Pogba and Tevez should find that final pass, because Marchisio offers them lots of opportunities to do it.
Exactly, operating with spaces is the prime strength of Marchisio. Finding spaces to exploit running into them and also, less visibly, closing down space defensively is his job, which he does excellently most of the times. Even the Vidal goal today had its origin in Marchisio running behind the defense making it possible for Ogbonna to play the (perfectly executed) pass.
Furthermore, whith his runs he also creates more room for the strikers to operate in, confuses the opponents defense and just generally wears them down.