You could, and you'd be right, but then why would either camp ever use the DF? JP servers use DF for clears and PF for practice. Why? Because there's an expectation that when you've entered a group with 7 random players, you aren't going to be there to waste anyone's time. The players with knowledge of the fight are going to perform at an average level and handle mechanics properly (or at least try to), thus either causing one or two wipes by a mistake of memorization or not causing a wipe all together. The new player, meanwhile, has no idea what mechanics will come in that fight and can easily cause so many wipes that the group disbands. Unfortunately, both types of players will enter DF, and there will be a conflict. There's nothing you can do to prevent it, it's a flaw within DF itself.
This isn't even really a problem with the players though, hard content where you can actually get the group wiped infinite times shouldn't be in the DF. This is largely a mistake on SE's part and, though I don't see it changing, I'd like for it to. Neither the experienced nor the inexperienced should be able to queue for stuff like Titan EX - they should have to make parties in PF for their preferred group comp and learn or farm the content from there.
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