Well, the pointing to the middle LB (Mike) isn't exactly because he's specifically all that important, but because he's the guy you start with to identify where all of the other positions are at. If I'm the TE and I can only see 2 LBs from where I'm setting up, it'd be nice to know which one is the MLB so that I may seek out the correct guy to block.
Missed assignments happen a lot because the assignments, at least along the OL, are difficult to begin with. Often times, the G and T need to block the DT and get up to the 2nd level (LB) - and where he's going to be during the play, not where he is at the start. Sounds fine, right, unless the DT is in a 3 technique (outside shoulder of the G) and firing out into the gap between the T and G. Sure, the T can block down on him, but as the play progresses, that T is trying to give him to the G to continue blocking himself as to free up the T to get to the LB.
Or a simple reach block where that G has to get his head across that DT and block him by himself as the ball comes up right behind him...
I'd never want to play QB at an elite level. You've got man (if the defenders are good enough) all kinds of zone looks. You can have zone behind man or vice-versa. A corner blitz with the rest of the back 7 rotating around during the play....safeties cheating up before sprinting 30 yards back or vice-versa....it's nuts.
And any one little thing can make a perfect play become a crap play....a throw that's too soft or too low, a lazy route run, a RB cuts back too early or too late, an OL doesn't get to the 2nd level, etc....
It's hard to get 11 people doing a perfect job simultaneously in any walk of life, much less when there are 11 people working against your 11. And even when you have all 11 doing their job, you have that safety you mentioned recognize something during the play, and blows it up. That's just film study. The DE who identifies a screen before anyone else can ruin it. If a LB knows your tendency of going to the quick slant on his blitz may just cancel it and get an easy INT.
It's a hard game with lots of through behind it...and if you're caught thinking during the play, you're toast.