Then make it optional. No reason not to have it. The raid is pretty easy after someone in your group has run it at least once. Odds of using mm and getting 6 newbies is near zero by now. Maybe only turn mm on after a month.
Firstly, the odds of getting all newbies would be pretty high for the raid. The raid completion levels are still very low, I'd imagine your average player hasn't even tried it yet. Add MM and you'd have a massive influx of people trying it for the first time. The fact many of us here have completed it means very little. Forums like Gaf are a very tiny percentage of the active Destiny playerbase.
There are lots of reasons against the idea of a traditional MM for the raid.
The obvious ones being it would open the mode up to AFKers and griefers.
It could also sour people's experience of end game content if they have a few MM games in a row with issues such as obnoxious/rude players, griefers, afkers, players without mics, no experienced players making for a long frustrating run that doesn't get anywhere, etc...
Of course there will be times when MM groups pull it off, and I'm not suggesting we should mother players. What I am suggesting is that a traditional MM system for the raid is not the best option.
What we should have instead is a party board in game. A place a person can either make a group and ask for people to join or search for groups. You could add criteria like "beginner's welcome" or "experience only", "mic required" etc...
This would benefit new players much more than MM as they would know for sure what kind of group they were getting into instead of throwing people together blind.
One huge problem with MM is its very much something a player will click on a whim. People will use it without considering the commitment required.
A big part of the raid is learning how the team your with works as much as the content itself, so if players were dropping in and out of a raid over the course of the multiple hours needed for your average run then it could get very frustrating.
The little bit of extra effort required to search a party board and sign up for a group would remove those who would be clicking MM on a whim without really considering the commitment required, and even if they didn't understand it could be explained to them by their team before they leave. The Raid should never be a "drop in/drop out experience".
MM works for certain mods. PvP, strikes, even Nightfall would be fine with MM as long as there was a disclaimer along the lines of "this content is recommended for a full fireteam with voice communication".
For the raid, MM remains a bad idea even if it were optional. A party board system would support players of all types much better and would help cultivate a community much more than MM ever could.