To take it to the extreme, consider Switzerland, where the largest party has only 26.6% of the votes. 11 parties are represented in the parliament, and the executive power is shared equally among 7 ministers representing 4 different parties. The system offers an unusual continuity in the executive branch:
The Swiss executive is one of the most stable governments worldwide. Since 1848, it has never been renewed entirely at the same time, providing a long-term continuity. From 1959 to 2003 the Federal Council was composed of a coalition of all major parties in the same ratio: 2 each from the Free Democratic Party, Social Democratic Party and Christian Democratic People's Party and 1 from the Swiss People's Party. Changes in the council occur typically only if one of the members resigns (merely four incumbent members were voted out of the office in over 150 years); this member is almost always replaced by someone from the same party (and often also from the same linguistic group).
http://en.wikipedia.org/wiki/Politics_of_Switzerland#Executi...
For example, if neither party has a strong interest, certain government departments can act without proper accountability to the public.
Clearly the US has been a successful country, but there are several problems with using the US as an ideal model: the benefits of this success are not necessarily distributed in the best way. And, with the US's huge resources, it's not obvious that the choice of political system is the biggest factor in its success.
Within the same party you can have representatives who are both for and against nearly every important issue! In the UK they need to form a coalition (sometimes) to run the government, but when they have that coalition, pretty much the only things which get voted on are those which are guaranteed to pass. Or if you want real crazy, go look at 2003~2007 Israel, where coalitions would need to be formed from several radically different parties.
I don't see any evidence that requisite inclusion of diverse stakeholders lead to better outcomes for the public.
Oh! I absolutely don't think that.
The two parties in a two-party system (US; UK normally) are often poorly distinguished, straddling two sides of a dividing line but essentially at the same point. This is even worse for the franchise as voters don't have a real choice at all: just the illusion of one.