The two party system also results in dramatically greater stability, typically at a cost trade-off of dynamism. It's hard to change a two party system, so hopefully if you've got one of those, you have something worth maintaining underneath. Systems with lots of parties are far less stable over long periods of time by contrast and are prone to rapid change and takeover. Australia for example, with its 13 parties with parliamentary representation, lately can't keep a leader for more than a year or two, with six prime ministers in a decade. In that extreme case it's causing policy stagnation however, as none of them are managing to tackle big, urgent problems before they're tossed out. Europe's typical preference (excluding a few countries like Russia) toward lots of parties has also resulted in neo-Nazi groups acquiring increasing government power and representation, another downside to that approach.