i don't get why people complain about it being so scripted and linear. i'm not sure i can name an fps that is non linear, and very few are any less linear. half life series, doom3, mohaa and all its sequels, quakes, every single fps almost is just as linear as this game. scripted events happen in all games, and help to shape the feel of the game, without scripted events it would mostly suck. infact, even the less linear games, like oblivion and the likes are chock full of scripted events.
as for more people pilling in to help control points of the battlefield in an attempt to keep the enemy out, you'd be really miffed if you got drafted in world war 3. "but captain, its really irritating me, they keep trying to win the fight, its so unrealistic, surely they should just give up when the first man dies?".
personally i find the spawning till you take area's fairly realistic, doesn't work in every instance because, on some maps they come out of area's that could easily be reserves coming to the front lines. but on some they do appear to come out of no where. but ti would become a very long task of map design to make every single area logistically capable of providing support.
how many times in these games, all of them, do you have missions where WE play as a character who is pushed into the front lines where people were ALREADY fighting before we got there, WE were the reserve fighters pushed into battle to help keep control of a point and fend off enemy attacks. almost every war game has missions where you are this reserve helping, it makes complete sense that when you attack they bring up extra people to defend. thats war.