Ex primals are incredibly different from just regular duties. Some that are not even relevant anymore still have mechanics that, if one person messes up, can wipe the entire group (Thordan Ex, Sephirot Ex).
The purpose of individuals making a PF for a farm party is to run the primal as many times as they can, as quickly as possible, with great DPS and limited mistakes. When individuals want to teach, they join Clear/Learning PFs. I've seen it all the time; I do it. But when people want to farm for weapons or for mounts, having a new player sneak into the party is going to prevent them for achieving their goal: to farm.
Ex primals take a while to explain, and usually there have to be multiple wipes for people to finally understand "Oh! That's how you do it!" or "That's what I was doing wrong." Most parties that still attempt to farm after helping a new person clear rarely seem to even get the clear in the first place. Because the new player dies to mechanics a lot, people get impatient, and they drop the group.
They took away the one feature that could guarantee no new players could sneak into farm parties by removing the Ex primals from Raid Finder to Duty Finder. I can't tell you the number of times I had people sneak into parties I joined, and we got nowhere. Then the group dropped, I'm without a single successful run much less a mount, and then I have to start/search for a new farm party.
If they want to move the Ex Primals and Savage into DF that's fine. But please give players the options from Raid Finder: Duty Complete, Duty Incomplete, Practice, etc..
EDIT: People keep saying that everyone's time is equally important, and that players that have cleared content should be considerate and accommodate those that haven't cleared, but it also goes the other way. New players should respect the wishes of a FARM party by not joining in the first place. Farm parties are not the place to get a clear.
People say that farm parties need to make PF because that's what it's there for. Guess what: it's also there for people to make CLEAR and LEARNING parties. It's there for all groups of players to use. DF, RF, or no RF, I would still want the option for Duty Complete or Duty Incomplete. This isn't even something that affects me (I have all the HW mounts), and I'm still for it.
I'm a mentor as well, have been for a while. If I want to teach, or feel like helping players clear content, I look for clear parties and join them, offering and and all advice I can. Some days, I don't want to teach (those days I just turn my crown off). I want to farm. Or get things done. And it's easier to get through content (usually) with those that are proficient in the fight (i.e., have it on farm).