The reason being, players would simply AFK @ a node instead if this was put into game. Simple truth.
IB 27 pages of rebuttles.
Which in reality probably is better than losing people and having to play a guy down for the 30 secs it takes for a replacement to load the map and spawn, assuming that the system instantly finds a replacement. If you have a guy standing there, even if he's AFK it gives the enemies the impression that it's guarded. Not to mention that it's an extra target so that those who actually still are playing arent getting hammered.
And I know how some people like to claim that they're turning around numerous games with the help of replacements. They might turn around the few games in which someone drops in the first minute, probably because of other reasons than just "you suck and I'm quitting", but they're sure as hell not turning the "standard" quitter games where people don't start to drop until it's in practice impossible to turn the game. I'm talking about 500-250 on CW, 4-0 in Huttball, all doors lost in VS in 3 minutes etc.
Also, it's not like people arent camping nodes already when it's starting to look like a loss. Some even do it the second the enemy team gets two nodes on CW, even if the score's still 580-570. Those are the people we should eliminate from MM.