I think the Multiplayer option is a lot of fun, and playing with more people and an Antagonist is a great way to develop one's skills. The rewards could use some work, but still worth it depending on how well we work together.
However, there are a lot of times where I just want to play Single-Player, enjoy the story, and not have to wait on players to respond/operate in an efficient manner. The only problem with Single-Player? No EXP/Level progression, and minimal/no rewards. Why offer a story with the chance to play with other characters, only to not let us play with the level based character unlocks? I understand that you give more rewards/EXP to incentivize Multiplayer, but what about those that get tired of Multiplayer, and waiting for Matchmaking?
Suggestion: Provide EXP progression and rewards but to a lesser degree than what one would receive playing Multiplayer, but don't make it so low that you have to "grind for a year" just to get to level 15 or so.
That's really my only major complaint with this game, other than that, I really love it and want to experience the story more! It just isn't really worth it if there are no real reasons to play it only to just get a few characters unlocked, that I may never want to use.