For people that want a little background on Fenix.
He was a BW player who played PvT, TvP, and TvZ. He got to the round of 8 in the TSL2 before being eliminated 2-3 by runner up Mondragon. He plays for FnaticMSI with Gretorp, KawaiiRice etc.
This is all off the top of my head, but I'm pretty sure it's correct.
If you're a high level player, everything every commentator would say would sound redundant. So what then, would you want no commentators at all? Its a silly argument. Their job isn't to teach you the game. They're there to provide entertainment, a human layer for the action.
No, they are not just there to provide entertainment. They are there to provide information we wouldn't have just by watching the action, such as harvester counts, food counts, production, resources, etc.
The caster also needs to follow the important action, since we can't control the live replay, the caster needs to do it for us, and their partner needs to provide information that is off-screen, such as a simultaneous drop or a 2-pronged attack.
Their role should be similar to NFL commentators. Here's what is happening (caster), and here's what that means (commentator).
Just to be clear, the person who controls the in-game camera is the observer. Most non-Korean casts give the observer a microphone and have them say some things too. That's not really the most accurate perspective though. Really what happens is that they don't have an observer and they make the guys with the mics do it. Ideally, the guy controlling the camera does not have to say anything at all but can listen so that he can take cues.