Ask PG: The right reason to take investor money

In your "Ramen Profitable" essay you wrote:

Ramen profitable means no more than the definition implies. It does not, for example, imply that you're "bootstrapping" the startup—that you're never going to take money from investors. Empirically that doesn't seem to work very well. Few startups succeed without taking investment.

So the question becomes--when is the right time, and what are the right reasons, to take invesment? In what situation is taking investment optimizing for success (for a web startup in particular)?