I had the same reservations you did when I first heard about it but am now drinking Dr. Lyte's Koolaid.
Here's one of the quotes that helped turn around my perspective: "We upgraded the matchmaking system to pair similar group comps together. It is possible that it might be harder to rank up in a premade group than as a solo because you'll be consistently exposed to crazier, and more competitive strategies."
There is quite a bit of dopeness to consider with the new system. More teamwork in Ranked games and such, less bullshit that poor Karl ran into when he made his last minute run for Plat. Maybe. We'll see.
But think about it this way: Solo Purist Pacov and Teamwork Makes The Dreamwork Thunder both play Ranked next season, you with only going in solo and while try and play with as many friends as possible each game.
For arguments sake let's say having a Dynamic Ranked group of 3-5 is a huge advantage over being Solo. Well... they're going to try very hard to match my premade against another premade of equal skill and size so where's the advantage I get if they manage to pull that off?
You'll be in 1/4 groups or 1/1/1/1/1 or 1/1/3 or whatever against the exact same composition so where is the disadvantage? Why is going Solo harder than having a premade five man team in this scenario?
This of course all hinges on Riot's goal of having Matchmaking place Dynamic teams go against other teams of the same size '95%' of the time. If that doesn't work out, then having lots of good friends will be a big advantage in Ranked.
Re: Adjusting MMR based on specific roles
Current Teambuilder Matchmaking is absolutely horrendous and any attempt to borrow from it's handicapping system would be a very bad idea. If you want to play Mid in Ranked, get good at Mid in Normals. Getting beat by better players is the way to get good at a role, specially Mid or Top. The new system will be in place so you'll get to practice the role most times that you queue up for a Normal Draft game.