As the title says, this trend is becoming more and more common by each passing day. It's an exploit that needs to be fixed immediately. I see it every time I'm a lone wolf and every time I party up. Matchmaking takes ages as it is, and with this, it makes the wait even longer. Worst is when I'm partied up and we get another party that leaves during the matchmaking lobby; guess what happens? My party gets split because there isn't enough players on the other team to "balance" it out. Chances are, if a party gets paired up against another party, one of them will be guaranteed to do this to avoid facing other good players. It's a cowardly move.
Asking for something to get fixed at this point in the game is almost laughable, but please consider fixing this exploit or at least look into it, Naughty Dog.
I don't know how you can really fix this, seeing as how even if ND does manage to fix all the ways of quitting out of the pre-lobby if someone wanted to quit badly, they can always rely on sign-out,pulling out cord, or quitting the game...
You can't blame ND on this one, as it is the players caring too much about their stats or not wanting a challenge.
I use this quite often when I'm up against a large party and it's clear that my own team is going to be weak. I honestly think it's funny that their party gets split up because that's the only way most of these guys are going to get a loss. Playing in an uber party of five is more lame than quitting in the pre-lobby so I don't feel bad about it. It's also worth mentioning that many of these parties are probably going to be trophy boosters that are just trying to find other boosters. You can thank the senseless DLC trophies for that.