I hate join in progress. It is ridiculous. I'd rather be able to back out and search and not worry about getting put into a game I'm about to lose.
Here's a scenario: You're playing by yourself, you search for a game. You get matched against a team of 4 and your team sucks. You've joined the game in progress and they're all noobs or they are AFK. Everybody keeps leaving the game so you're pretty much screwed.
You don't want to quit because: 1. You don't want to get banned. 2. You don't want to have to join another game in progress that you will undoubtedly be losing. 3. You're not a quitter and like to follow through on what you've started.
I've seen more people quit out of games in Halo 4 than I've seen in any of the other Halo's. I shouldn't be starting games 5v1 when the importance of the opening rush can make or break you. When I get teammates later on in the game, the other team can be in a near unbreakable setup with all power weapons, power positions, and even vehicles.
Also, I don't like joining a CTF game when there's only 2 other people on my team, who have 40 points each, haven't captured a flag and the other team have 3-4 flag captures and under 3 minutes on the clock.
There's a lot of AFK people playing because nobody wants to back out because they don't want to JIP and lose. Also, sometimes you start a game, then all of your teammates quit on you and you're by yourself for most of the game. JIP should be taken out.
This is why Halo 4 should have had a public beta. If I wanted CoD style matchmaking, I would play Call of Duty. By the way, JIP sucks in CoD as well.