Having MSN up top means that neither of the midfielders can be too attacking. If anything, the system would work best with 1DM and two CMs that are physical and cover a lot of ground. Coutinho is too attack oriented to play at Barcelona imo, is not physical and does not have the defensive abilities of a Barcelona CM at this stage and with that attack.
I personaly like a Busi - Gomes - Rakitic midfield and it can be a good solution for the future, provided that we play them together more.
I don't think that we need two Cms with MSN.
On one hand, you can never plan your long term future when you have Neymar in your team.
He could be gone in summer of 2017 or summer of 2018 easily.
So, on one hand, our MSN-pendencia system could change very soon and a different system, with a different CAM could work.
Also, about MSN and two physical Cms, I remember an interview when Laporta and Rijkaard bought Deco in a summer of 2004, Rijkaard later said: we didn't want a situation where we depend only on Ronaldinho and his playmaking abilities. He is a world class, but when he has a day off or when 2-3 opponents man-mark him, our team is in troubles since we don't have a playmaker anymore.
This why we bought Deco (who played as a Rcm/Rcam) on the opposite side of a pitch: if the opponents will use 2-3 players to mark Ronaldinho, we will have our 2nd playmaker in Deco on the opposite side of a pitch with a lot of free space to create something for Etoo+Guily/Larsson.
Anyway, I think that we could use a similar logic/idea even here.
Even though we have Messi and Neymar as 2 playmakers, when they are heavily marked, we are in troubles.
Imo, a team with 3 playmakers is again tougher to mark than a team with only 2 playmakers.
(Someone will say that we have Iniesta, but he is older and older and far from his prime).
I agree with you about defensive skills from Coutinho, but even Rafinha and Denis are poor in defense (and even a current 32 years old Iniesta isn't on the highest defending level anymore).
But again, a slightly more defensive fullback on Coutinho's side (let's say Alba), more defensive Busi and a Rcm who is a workhorse (someone like Rakitic or if Gomes will improve) and it wouldn't be that horrible.
Also, again, imagine if Neymar will leave and if we will buy a different winger, that would change a whole balance of midfield and attack and the way how our team attacks and defends.
I could be wrong, since Deco was here 10 years ago and I didn't read forums back then and maybe my memories of him are wrong, but how I remember him from Barca, he was never "that good" in defending.
He knew to cover and defend, but he was way weaker than Xavi in that area, imo.
Those were different times, but if we ruled Europe with 3 attackers and Deco, maybe we could do the same with 3 future attackers and someone like Coutinho.