I have found that PuGing can be extremely fun in ME3. Let's be honest, in a preset group there is practically no challenge anymore. Everyone knows how to play, everyone knows their role, so stuff melts. Join a PuG, and suddenly it's can be anything from a breeze to a horrid wipe. You get to deal with new sets of challenges (nothing like dying on the map/enemy/difficulty combination you know you can solo due to teammates 'helping'). Can it be frustrating? Of course. But its also can be very fun and challenging. As long as everyone has fun, its fine.
I'm with you there. Personally, I enjoy a mix of both kinds of matches. Having some matches just flow perfectly can be a lot of fun, but gets boring after a while. And trying (and sometimes failing) to pull a horrid group through a match can be a very fun challenge, but gets frustrating without some good matches thrown in in-between.
Now if you require manifest grinding (like ME3 did originally, and like DAI seems to do), add some artificial handicaps on speed of item acquisition based on character class combination, then yeah, I can see most people sticking to preset groups, which is honestly what I hoped Bioware would try to avoid.
I'm hoping the "deconstruct this item to get parts for another" thing is going to help a lot with that. I guess we'll just have to wait and see how hard it is to get some of those parts.
I believe you have a point about DA:I being less a problem. That problematic crowd usually don't like swords and magic so we're probably safe from them this time around 
You clearly haven't played enough MMORPGs.
"Yeah, I know YOU are the tank, but I want to run in and start the fight by myself, and then scream at the top of my keyboard-lungs at you for not saving me!" It's ALWAYS the dual-wielders...