Well, it's really impossible to say how 'changing the game' in this way would cause the system as a whole to shift. Assuming we're switching to a system where all of the geographic distribution stays the same, with proportional seats added on top of that, yes, the representatives would need to work together in order to get laws passed.

That doesn't make minority parties irrelevant. Just as the GOP and Dems are both relevant even when they're in the minority, even legislatures where parties may only have one or two representatives will work to bring them in to the fold. Giving them a vote gives them something they can give and take from you; if their issue isn't important to you, their vote may be. They can work to prevent legislation harmful to their cause (whether through parliamentary procedure, the press, or politicking with other legislators), and adjust legislation to be more beneficial, even if it doesn't completely shift policy of the ruling party.

This is more of a practical benefit to the people in those parties specifically; generally speaking, it would probably benefit everyone in the long run. People don't vote when they don't feel like their voice is heard--proportional representation ensures that everyone has a reason to vote, including voters whose political preferences swing opposite to their districts; Even with the two-party status quo, plenty of inner-city Republicans and red state Democrats don't bother casting ballots, because they know they can't change their regional result. Proportional representation across the nation can draw millions of votes for parties* that would stand no chance in a district-by-district poll. It encourages people to go out and try to have any amount of impact, rather than the current system where most voters are mathematically forced to pick the candidate they feel is less bad, or least damaging.

*generally, where proportional representation is a thing, people vote for parties, as opposed to American elections where you are generally voting for a person. The party would have a list of proposed legislators, decided by processes similar to primary elections, who would be appointed first-to-last until the party's allotted seats are filled.