Here's your answer.

So whats the advantage of partying if your over 100 paragon? Exp gain is nice but if your top 100 then what? I also saw about gf/mf + but is it really gonna make a difference?

@Mehsiah It's more about not penalizing players for running multiplayer than it is about rewarding them for playing multiplayer. Right now, with the exception of Ubers, there's a definitely penalty playing multiplayer in terms of efficiency. Players have to make a tradeoff of 'fun' for 'farming.' For many, not all, players, farming is synonymous with fun.

tl;dr: Coming from a Barb's perspective, you're completely right that multiplayer is pretty much pointless.

You can think of the 10% XP multiplayer bonus as a way to help make up the gap for the inefficiency that comes with playing in a party. When two people attack the same monster, but only 1 attack was was needed that's a wasted attack. That wasted attack would never happen in single-player.

The 50% increase in HP vs. 70% now, makes much more realistic scenarios where a party can collectively play at a higher MP level than they could individually. Here's a hypothetical example where it would be advantageous to farm in a pair:

One hero does 100k damage per swing encounters a monster with 75k HP. It takes one swing to kill it on MP0, but with the a 50% increase in HP moving from MP0 to MP1, that monster now has 103k HP and will require two swings to kill.

Two heroes each do 100k damage per swing and encounter that same monster that previously had 75k HP. The game adds 50% HP because there are two players in the game so the monster has 102k HP. Each players has to swing once to kill it (dealing 200k damage in total).

Those two heroes create a game at MP1 and encounter the monster. It has 225% HP for a total of 169k HP. Each player swings once and they kill it. They kill it in the same amount of time it would take to kill it soloing MP0, except now they get the XP bonus for playing multiplayer and the bonus for playing on MP1.

As a Barb, it's always going to be stupid for you to play multiplayer. You need Fury to maximize your dps and Fury generation comes almost exclusively from Battle Rage. Sharing targets with another player will generally mean a decrease in your Fury generation. It still works, but not nearly as efficiently as single player. Some Witch Doctor builds are in the same boat.

The devs recognized that Archon wizards can't play multiplayer at all and made a big concession to them. The devs very well may do something else to help Barbs. One solution that comes to mind is a passive bonus to Fury generation. It still wouldn't be as efficient as playing alone, but it would help.

Since DH's get their bonus Hatred Regeneration from The Bat Companion or picking up Health Globes, they really only see the benefits of multiplayer. With the exception of Sweeping Wind w/o a 4-piece Inna's Set, Monks are in a similar situation since they don't require kills or strikes for their Spirit Regeneration. Witch Doctors have some abilities that require strikes to keep up resource generation, but it's not nearly as hobbling as losing Fury generation for Barbs (or archon kills in 1.0.7).