...and what prevents a first-timer from making a learning/clear party of their own instead of sabotaging someone's farm run?
Sometimes you can't find a party of 8 from people you know. I know there's plenty of times when I've only been able to scrape up 4-5 people and we end up pugging the rest. So... that means you should have to endure first timers mucking about and messing up your farm runs? I don't follow your logic here. If I put up a PF for a farm, that should tell people that they need to know the fight. If someone sends me a tell saying they've been to x% and just haven't been able to get a party to clear it, I give them a fair shot. 2-3 wipes is usually enough to tell if they actually do know the fight. If it's clear that that's not the case, I tell them they need to get some more practice and ask them to leave the party (if it hasn't already broken up due to people getting frustrated). It's terribly selfish and disrespectful to jump into a farm run and expect the other seven people to carry you through the fight.
Then you're getting into crappy farm groups whose leaders need to learn to kick weak links. :/
