Others commented about checking in, but here's a couple of real issues which happened to me on this trip:

a) Baggage allowance, as a 1P my bags and my companion's bags should be free - instead we get hassled by the checkin counter for a $160 (??!!!) fee to check my family's two additional bags!!!!!

b) If the flight is full, there is often "no way" to get your minor children re-seated next to you - picture a 5yr old in E- middle 10 rows away... and no-one who has "paid" for E+ is going to be bumped to that E- middle without .....ing and moaning about it, so the agents tell you to deal with it at the checkin, and the checkin agent tells you to deal with it at the gate, and the gate agent tells you to deal with it on-board and the stewardesses say they cant do anything.... really? Thats how 1P gets to sit E- middle when everyone was assigned E+ together (plane change and automated sweep took care of that for me, soooooo helpful arent they).

So a single PNR is essential when you have minor children - thus making eCerts nearly worthless if you have minor children... so I'll agree with the posting above that United intends breakage here.