Many here focus on moment of flash rather than look at the entire performance of a player in context with the team. One of the most important aspects overlooked is positioning and reading of the game, and that is a quality that 1) some players are naturally far more savvy than others, and 2) is a skill that improves as players mature. You simply don't often see it in young players. Thiago was an exception, as he was clearly a talented student of Xavi in that regard, trying to develop as a CM in addition to his already considerable skills as an attacker.
Rafinha shows many great flashes of attacking skill here and there, but he went entirely missing in his position in the first half against Elche, leaving Alba and Neymar to attack on the left on their own. Sergi Roberto is a solid strong player, but has a tendency to drive the ball into dead ends. And so on. Not anywhere close to Iniesta in reading the game, yet people only focus on the quantity of individual attacking moves, rather than evaluate how the players function in the system.
With Iniesta or Xavi, you get masters of reading the game and positioning. You already have three attackers in Neymar/Suarez/Messi, and if Rakitic joins them, it's safer to have someone in the midfield helping glue the team together. Iniesta has been sitting a little deeper to allow Rakitic to attack, and will still occasionally see the chip over the top or through ball for Neymar/Alba, or cross for a run at the back post. With Rafinha, assuming he plays with the same lack of positioning he did in the first half against Elche, you risk a gaping hole in the left midfield for Busquets to cover by himself (until the chipmunk can catch up getting back). Might get away with that against Elche, but beware that space against sharper counterattacking teams.
Iniesta/Xavi and Rafinha/Rakitic/Roberto against strong teams. Only one of the "R" players at a time when Barça can't afford the risks of less positionally aware and disciplined players.
