That is totally besides the point. Sure he needs us to win trophies. Everyone needs us to win trophies, since Barca win most.
But the point is that Cesc makes the team stronger and more complete. Forget about tonight's easy match...with Iniesta out for a month or so, our team would be heavily exposed as happened in the past (plus extra pressure on Xavi) In 08/09 in the Jan to Feb period when we dropped several points in La Liga and also were under par in the first leg Last 16 game against Lyon. Same happened when Iniesta was injured in the closing stages of of 09/10 season when we unceremoniously lost our European crown to Internazionale of all teams.
Cesc (like I have repeatedly stated) is our guarantee that even if one of our three little midfield geniouses is injured, we will still hold undisputed midfield sway.
And after all, what's so wrong about having the top 3 midfielders in the world under one roof?
I think everyone agrees with that. Cesc has only made the squad stronger in many senses: he's a fantastic impact sub, he can cover for Xavi and Iniesta and he brings some qualities that our other current midfielders don't have (eg. he's more direct than either Xavi or Iniesta, and a better goalscorer), not to mention he has such great chemistry with Messi.
But saying we NEEDED him seems to suggest that we'd struggle without him, which is not the case at all. There are a handful of players who would make Barca stronger/deeper, which doesn't mean we need them all.
Also, I'm not totally convinced that Cesc is the third best midfielder in the world, although I hope he proves it this season. So far, 4 goals and 4 assists is a brilliant return, hope he keeps it up.
And I don't think Cesc can fully replace Iniesta. Andrés is a once in a generation player imo and the best midfielder in the world. There's no way one single player can replace him, it will take a collective effort. As good as Cesc is, what he has to offer doesn't replace Iniesta's dribbling and darting runs through the defense. Especially in big games, I feel that Iniesta is our most important player along with Messi. Sure, it's better to have Cesc on his place than Keita, but it's still not the same.