Pepe was our left winger for the first few games of the season. The only reason Marchisio is in the starting line-up is because Del Neri knows it would be such a waste to put him on the bench. He played the guy out of position just so he could start. And if we didn't have any left wingers; I'd bet you my left nut that Del Neri would play something of a 4-3-3 as long as Marchi is playing.
Like I said, he hasn't been super in every single game this season but he's been great overall this season. He's been playing out of position and doing a very good job at that too.
Pepe was our left winger, yes, and though I think he did an adequate job, apparently he proved himself expendable and was dropped. It wasn't as if Marchisio forced
him out of the team with some imperious form, but more so because Del Neri didn't think he was up to snuff and truly viable alternatives don't exist for us (Lanzafame who's been amateurish...and Marchisio).
The only reason he's in the starting 11 is because he did an admirable job against Inter "on the flank" (a tactical choice for helping to control the center midfield, as he hardly got wide).
Regarding your last comment? Huh? Why aren't we playing 4-3-3 now, then? He's sticking to a 4-2-2 despite not being able to count on Lanzafame, Pepe or even Martinez.
So, really, if we were to have a proper left winger --> Marchisio would be benched. Simple as.
-edit. The irony of all this is when he played in his favored CM role he was often invisible, yet now, out of position, he's had some good games.