I'm not sure who Microsoft's first investors were, but consider this. "No startup can be the next Microsoft unless some other company is prepared to bend over at just the right moment and be the next IBM." - http://www.paulgraham.com/gh.html
(http://paulgraham.com/bronze.html)
Now that I know more about the history of Google, I wouldn't phrase it so extremely, because it's unfair to Omid Kordestani and Salar Kamangar among others.
Here's some video on the subject: http://msstudios.wmod.llnwd.net/a2294/o21/presspass/BillG/Ma...
A history of the personal computer: the people and the technology By Roy A. Allan
Was that thing vital?
However, the question is whether YC can help or not. Could the Google founders have benefited from YC? Could Zuckerberg? In retrospect it was unnecessary, but that couldn't have been known at the time. And I think the answer to that question is certainly yes. All the benefits Martin enumerates in his essay apply equally to people who become very successful.
In such cases, where the outcome is highly variable and very sensitive to the inputs, it's wise to have as many good cards as you can get. You can't just bet on one thing; every bit of support or advantage you can get could be the thing that makes the difference. (Sorry for the weird mixing of metaphors.)
1) Innovation is the wrong word: Facebook, Google, and Microsoft weren't "innovative" ideas. Social networks, search, and software were all things that people were already doing. They were successful due to a multitude of factors including talent, ability to execute, and timing.
2) Those three examples aren't YC companies, but the two more recent ones, Facebook and Google, both had to raise VC, which means convincing people that their idea is worth investing in. At no point was it obvious.
People are missing my point; it's ridiculous to say a blanket statement like "Google, Facebook and Microsoft did it without YC, what say you, entrepreneur!" when you're evaluating them when they are already billion-dollar companies. It's the same thing as someone trolling to say "I don't have to go to college cause Bill Gates didn't!" So when you're evaluating a company that is already successful, of course they won't need the help because their products are already so compelling.
Now if you say "that's not why you're getting down-voted...these products weren't born awesome, they could have used YC then" well then this argument is still flawed. Why didn't Facebook need something like YC? Well it was Feb 2004, before YC even existed. None of these companies HAD a YC to use, so it's not a comparison you can even make valid. Incubators didn't exist for MS, Google, Facebook, Apple, AOL, Netscape...
Of course these people needed to hustle. But they definitely sold themselves - Microsoft was bootstrapped, and Facebook had 100,000 members in 3 months until Peter Thiel caught wind of their efforts and approached THEM. Can everyone use the help? Of course. Facebook did it without YC and could do it again without YC. 99% of entrepreneurs are not in that situation.
As the entrance barriers for internet innovations become lower and lower, the competition to attract publicity and secure critical mass has become more intense. To me that is where YC offers the most value.