As the title says, I feel this option should be toggled on by default. In lower level dungeons especially, it can be near impossible to find a replacement for someone who leaves. Tanks are practically impossible to find, DD's almost impossible, healers, so-so. No one checks "Join party in progress" because they want their experience points or whatever. The system punishes players who have someone bail on the party for whatever reason. It also punishes DD's who check it to speed up their queue(A sensible choice) by way of possibly joining a party thats missing more than just one person. This situation forces them to sit in a 3/4 group(Previously 2/4) waiting on a 4th person who probably wont come, while the time limit on the dungeon ticks away.


TL;DR I'am suggesting not even giving players the choice, just having them join either a fresh party or one missing a member all the time.