I can give a whole bunch of examples. Apple: Steve Jobs and Steve Wozniak. Microsoft: Bill Gates and Paul Allen. Valve: Gabe Newell and Mike Harrington. Id Software: John Carmack and John Romero. People split for various reasons, sometimes due to disagreements. Sometimes personal reasons or change of interests. In the end most companies seem to end up being run by one person whether or not they were founded by more than one.
There are also successful companies that were founded by one person, like Dell and Amazon. So to me it seems that statistically speaking eventually you'll split up and one of you will have to leave the company.
Ultimately it seems that the co-founder issue is mostly an issue of initial mutual support. It's tough to go through the early stages on your own, and having someone to share the workload with and get moral support from increases your chances of success. But it can only truly work in the long term if you decide at the beginning that one of you will be the president and have the final word in major decisions. Otherwise it's a recipe for trouble down the road.
Finding great people to be employees is very important, and I'd say more important than finding a co-founder. In a way your co-founder has to embody everything... but each employee can contribute a part to the whole.
Woz never wanted to run a company, he just wanted to build stuff. (Shameless plug for Chapter 3 of Jessica's book, Founders at Work).
For every example you cite, there's probably a new story line.