Being able to join quests in progress, excluding Government quests, is always unstable. Not only that, it's borderline cheating.
You could join a quest that is in progress, right near the end, and get the reward meseta and/or items. Something that could be exploited if you had multiple characters as well. At least with government quests, there are flags for each quest so you can't get multiple items and rewards, but this isn't the case with a number of the regular quests.