Um.... No. "Farming" implies in itself that you've mastered the content in question to the point where you can go in and one-shot it, or at least down it with just one wipe in case of accidents (since they happen now and then, even when you've done something dozens of times).
Yes, clearing/downing end-game content is meant to be hard. Learning the content (after clearing it) to the point where you have it on farm takes time. But a farm party implies that the people in there actually know the content. It's not really for the people who cleared it once and are still amazed that it happened. A farm party's main target group is the people who have cleared the content multiple times, and who can go in with the expectation to clear it every time -- provided the rest of the group is at the same level. It's intended for multiple, quick runs, to get loot for as many people as possible.
True, some people who join -- who have cleared the content -- are still not at that level, and people who have bought a clear while lying dead or sitting outside the arena wouldn't set off the "bonus run" message. And even with this kind of system in place, they'd be able to get into the farm party, but why would it be really bad to bar new people from joining?
Personally, I would see it as a good thing. It would mean that there would be more practice- and clear parties, since the people sneaking into farm parties wouldn't be able to do that and would have to make their own (or wait for someone else to do it). It would mean that said practice and clear parties would in all probability fill up faster. And it would mean that the people in those practice and clear parties would learn the content more in-depth than they would by being carried, and once they had their clear, they'd get to 'I have it on farm' a lot quicker than if they were carried. Sure, the entire process would take longer, due to the beating-your-head-against-the-wall, but... I can't say I don't see that as being fair, and that's with me being late to end-game content, barely having started second coil by the time final coil came out, and I still haven't set foot in Alex Savage, nor beaten final coil (since it's a side-project in my FC/static).
I don't agree that it would take hours to fill a farm party, if the people joining it could be assured that there'd be no first-timers sneaking in there -- it's one thing that has sometimes kept me away from joining -- but to be honest, I'd rather it took longer to fill the party than fill it up relatively quickly only to get in and realize at least someone is trying to get a ninja clear.
And like people have said, they have sometimes said, "Fine, you can stay, we'll teach you the fight." Only to have the newcomer afterwards say, "Thanks for the clear, I'm out!" instead of staying for those multiple runs that the PF listing indicated.