Well, Juventus were only in two competitions this year. Next season, with more games to be played and if injuries start to pile up we could see alot more of Marrone.
Not to mention, Marrone is more of a backup to Marchisio/Vidal, where there is competition (i.e. Giaccherini, Padoin, etc) Verratti is the only true alternate to Pirlo, and given that Pirlo is 32/33 years of age now I don't think he has the legs to play 90mins each game with 3 competitions.