That's how it's always been up to now, and it's what I'd expect.

: Though it also led to a situation where instead of quitting, people would
: feel it necessary to instead go AFK in H3, Reach, etc.
: That also compromises the team, having a sitting duck lingering around for
: however long.
: Now, instead of going AFK, you can easily leave the match and your place in
: the team will be filled by someone who is able to actually play at that
: time. The shift in players per team is only temporary.

Sometimes it takes a while to get new players in though, and sometimes they're worse than the ones that left.
Anyway it doesn't seem to have had a big effect on people idling/being AFK/AFC. What really gets me (and I meant to bring this up weeks ago) is for some unfathomable reason they took out whatever it is that boots you to the lobby if a game ends and you haven't been playing the last few minutes. I know because I had to go do something while I was in the middle of a game, so I left, thinking if I didn't make it back soon it would put me in the lobby and at least I wouldn't inconvenience anyone else, but when I came back I was in another game, and when I asked my sister, who had been nearby, about it, she said the game ended, I was put in another game, and then that game ended and I was put in another game. I don't even want to think about how many "players" on Halo 4 at any given moment are just controllers sitting there.
If someone quits before the game even shows the other players, I think they should be replaced. If someone quits before the map and gametype are picked, I think they should probably be replaced. If someone quits while the countdown is going or while the game is loading or maybe even while loadouts are being chosen, I'd be somewhat OK with them being replaced. The join-in-progress system Halo 4 has means players may or may not get a choice in what they play beyond the playlist, and I have a fundamental problem with that.