I'm really on the fence about this because both arguments have truth to them. If you look at many successful companies, they were often founded by two people. However you'll also find that usually one of the founders leaves eventually (usually after a short while compared to overall lifetime of the company), and the other stays to run the company.
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.
Right, and that mutual support can come, in large part, from other employees. One company I worked at, due to the personality mix, was kinda rough, and we were in a tough time funding wize and market wise-- but the office manager, and older german woman with a strong will, took it upon herself to fix things and thru random help, negotiation and general positive attitude really made things a lot better- she was a one woman support organization for a 12 person startup.
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.
Hey, don't forget that Paul Allen left Microsoft with cancer. Once he went into remission, he didn't go back to work, he decided to take care of himself and put his billions to use. Pretty easy decision, IMO.
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.