I really like Marchisio and I'd love for him to reach the level where he's worthy of our armband, but to be honest the criticism of him is valid.
Sure, he's been played out of position for most of this season, but even when he played in the middle, he'd go missing for long spells.
Like you said, he's very technically gifted and barely puts a foot wrong, but I often look out specifically for Marchisio when I'm watching our matches, and he just doesn't impose himself on the game enough.