It's not to play all games, but as a plan b. In a game like today it could be helpful. Alexis and Pedro didn't do much, so I'd drop one of them for a 9.
Again, why do you assume they would have so much trouble marking him? The only times today we were able to link up successfully between the lines was when we had intelligent movement from one of our midfielders or, what do you know, wingers,
into that space; a number nine wouldn't be able to achieve this because he wouldn't have anywhere to move into since he was already there. Half of that clever linkup play is due to players moving out of their position, cleverly interchanging, drifting into dangerous areas from places where they seem harmless. The markers don't pick up on them because they aren't expecting anyone to be there; if a number nine was there, not only would he be marked but the defenders would be more attentive in general, since someone is in and around them constantly. Just look at some of the runs Cesc and in particular Iniesta made. That late play that was almost a goal for Messi, that play where Iniesta had one defender on him and should have shot instead of lay it back... All those plays were because Iniesta moved into the space that a number nine would occupy and the defenders
weren't expecting his arrival.