All the justification for joining in progress is a pretty cute way to say "I want to be carried", and it doesn't even matter because apparently in some people's mind, you'd have a guy who click 'join in progress' checkbox, and then say it pops up AK with 2/3 cleared, that guy is going to say, "This means they're at Ananatoboga which means a smooth sailing to an easy 40m stones. I better NOT join this party!" Anyone else who sees the "2/3 AK completed" will easily realize this is most likely an easy win and take their chance (there's a slim chance say everyone ragequit at Ananbotoga with 5 minutes left but you got to take your chances). Since the DF is still a first come first served deal, having a 'join only groups that already did all the work' wouldn't do you any good, because anyone else can easily see when a group has done all the work and you wouldn't get these freebies any more often because everyone ahead of you in the queue will definitely claim those freebies.