Personally I think people trying to practice or get first clear should be using PF, just like how in the JP data centers most people use the "simpler" (as in not being able to choose who you'll be grouped, how far they've progressed in the fight) DF queue for farms, because farm parties need much less communication than practice parties or aim to clear parties. You just use macro to outline the strategy, people call out their spread positions, priorities etc, that's it. Everyone knows the fight. This works even when there's a language barrier between the Japanese and English speakers in JP servers, as long as we use google translate to get rough understanding on the macro.
When we're talking about practice and aim to clear parties, there are a lot more variables involved. Are you practicing for the early phases? Or are you trying to clear because you've practiced everything up to the last phase of the fight? Are you going in blind, not watching/reading any guide? Are you trying a non-standard strategy? You have absolutely no control in DF, unlike PF where you can specify if you're practicing certain parts, using different strategies, doing blind runs for challenge etc. You can't just mix everyone aiming for different goals like that together and hope they'll somehow work it out and clear the fight.
This probably explains why you're thinking this way. For the harder contents in this game, helping new players doesn't mean clearing in 30 minutes instead of 15 minutes. It may mean NOT clearing at all in 90 minutes. Some of these fights aren't so simple/easy that you can learn the whole thing in 60-90 minutes, so if you want to farm the content, having even one person new to the fight in your group can completely make it impossible to clear. It's not that people don't want to help others learn, but sometimes helping others learn means not clearing the fight.