Re: 2 party system benefits and drawbacks...
What benefits?
Eliminates the possibility of vote-splitting which allows someone to come to power with far less than 50% of the vote. (And yes, there are ways to alleviate that problem... runoff elections, ranked ballots, etc. But those add to the complexity of the election.)
A 3+ party system may cause the formation of coalitions. The problem with those is that it gives undeserved influence/power to smaller parties. Plus, voters don't really have the option of selecting 'coalition' on their ballot, and may not have been in favor of the party they voted for "linking up" with some other party.
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I see only drawbacks.
-easier to gerrymander
-unrepresentative and marginalises the majority of voters
-allows negative campaigning
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Negative campaigning is more than possible in a 3+ party system. Canada has 3 or 4 national parties and there are plenty of attack ads that get used every election.
Actually I think the opposite is true. You're more likely to get partisanship in a 3+ party system.
When you have only 2 parties, your congress-critters may differ in ideology enough from the party leaders that they sometimes vote against the rest of the party. In a 3+ party system, politicians can find a party that better suits their ideology, so they would be less likely to go against party leadership.
Now, it is true... the U.S. has become hyperpartisan, with many issues decided along party lines. But it wasn't always that way, and if you go back a few decades you will find much more flexibility in the way congress-critters voted. On the other hand, politicians in Canada's system pretty much always stick to party lines.
Democracy in the US (and in the UK) is dysfunctional and needs to be fixed.
A bit strange that you talk about how Democracy in the UK is dysfunctional in a post about 2 party systems, since they have more than 3 parties, and while 2 parties dominate, at least half a dozen have elected members in parliament