I love it when people leave early, allowing me to join in progress. The loot I want is mostly dropped by Cerby, and if I join in progress after Cerby, chances are high that I'll get locked out from the final fight. That lets me go watch netflix for 5 minutes and get free light, 40 sold, and 20 poe for doing so. Totally worth it. The final fight is really fun too, especially when you screw around with BLM tricks (manaward to take nothing from meteors, aetherial manip if you're in the middle when she teleports, scathe with crazy spellspeed and UI up to tear through the Shadowlurkers, etc).

When people leave in progress, they're generally the people who are less dedicated to the run and generally worse players than the people who replace them. Most people that I've seen farming the first or second boss for gear to use in FCoB ignore mechanics and just try to burn everything down and drag out the raid by doing so. I love it when they leave. Also, over half of the people I've seen doing it are BRDs who refuse to sing, making them just low DPS DPS. Four BRDs one run - no foes, no ballard. Three of them left after the first boss (farming for the BRD hat, which is really nice (crit main, acc second, much much better than Ironworks (acc/det) or Dreadwyrm (ss/crit))).

The run went much smoother after that point.