I can answer from my perspective:
> The organizing and wait times involved to play a 2-3 hour game mode is additional overhead on top of that 2-3 hours of playtime. From my MMO experience, it can take a good while to get everybody together and ready to go, and that was among people I knew.
> Looking for players to group up with among the pool of GAF isn't much different than matchmaking except the pool of players is much larger in matchmaking, the entire process is automated, and underlying metrics can filter that matchmaking by connection quality, skill level, or quit rate. I also don't know the difference between a random on GAF and a random in matchmaking.
> Playing and failing is still playing. Doing logistics to put a group of six players together is not, and unnecessary in 2014 when matchmaking is available on all platforms.
It isn't even about having six friends, or having six friends who have Destiny. Its about having six friends that are max level, online at the same time, want to do a raid, and can set aside a few hours to try it. That puts you into "join a clan" territory, and anybody who is not used to the mindset behind MMO raiding and the gear hunt in end-game is in for a nasty surprise.