When you're focusing on growing your user base, its not worth it to lose a few users and generally make your site a little less usable and appealing for $100ish dollars a month.
There are so many variations in placement, eCPM, clickthrough, etc across different sites that the only real way to know whether or not something works for a particular site is to test it. Without this sort of testing, it's difficult to gauge how much you really have to grow to become viable.
Unfortunately, I think a lot of companies use this argument to justify a non-sustainable business model, in the hopes that once they get enough "eyeballs", the monetization will follow. So they don't put up any ads at all, then when they do eventually put them up, they are likely to be shocked at the actual (likely to be much, much lower) earnings.
Another advantage to putting up ads early is that users become accustomed to them from the start. Going from no ads to ads, especially after a community has grown used to the concept of no ads at all, can cause the same loss of users.