It's one thing sitting out because you have too many show up, but quite another to sit out because you're one or two members short of the requirement. No one is asking to enter this dungeon with 9 or 10 people. We're asking to enter it with 6-7 in a situation where 12-15 people show up. If 9 turn up then tough luck on the 9th, but the content should be manageable (although more difficult) with 6 or 7. The fact the developers are willing to consider lowering the requirement at all tells you this.