No, the main diffference is those who are actively seeking out groups via third party methods are much more likely to be commited for the time required to complete the raid.
Many people using MM do so on a whim, and people who play content on a whim are much more likely to be less commited. You gwet peopel dropping out of pvp and strikes often, if that happend during a raid it would be problematic. And it would happen, a lot.
It would bring many players who don't understand the time commitment, how by working with the same team through the entire encoutner to build the rapour and synergy required to efficiently take down each new section is almost essential, and that teaching/learning new players every time someone drops out and MM replaces could make the experience a chore.
It would also open it up to griefing much more as players would be more inclined to grief if they didn't have to go through the process of seekign a group and speaking with people and actually engaging with them personally pre-raid.
The issues that MM brings could also turn people off of raid content. If MM matched an entire group of newbies who had no idea about time comittment, difficulty, etc... it could potentially sour the entire thing. Right now a newbie can find a group with people who are actually commited to learning, or include a person with experience to lead.
MM is not always a good idea. Other games have done it, sure, but they do bring with it their share of problems. On occassion you will find that one MM group that syncs up and creates a great experience, and for experienced raiders who need a 6th man quick it might be a good tool, but for many it will cause frustration.
A much better idea, imo, would be to enhance the social feautres in game. Give people flags so they can mark themselves as "looking for raid", then a search filter so you can find willing players to group with. Maybe an in game party board where you can leave messages looking for the type of players you'd like to group with, proximity chat in tower, etc... ways for you to actively search for a group in game.
Players with limited time could use these feautres to plan ahead and find likeminded players with similar schedules, it would eliminate players jumping in on a whim and all the issues MM brings.
EDIT: I'd like to say this isn't about mothering players, but about presenting a way to prepare for the content that would require a player to actively seek out a group. This act alone requires a little more commitment, and is a good base for the commitment required for the content when you're a new player.