It's relevant b/c in both cases he's the deepest man in midfield acting as the pivot, fulcrum, or call it whatever you want. The roles are similar in the two formations.
And with Khedira and Matuidi, he certainly has to be the one in the middle b/c he's the only one out of the 3 who can play there.
Agree that Marchisio should start over Khedira, & in that case, he should assume the central role, but that's a diff topic & doesn't change Pjanic's ability to play there.
As for the passing argument, this role, which he's been playing since Jan, gets more out of his passing ability.