If a defendant speak in court, he loses his Fifth Amendment rights. Add onto that, and most defendants do not have any knowledge of the law, and can easily be tricked. It is even worse than that, most defendants are of below normal intelligence and education, and have above average emotions, so are very easily be tricked.But, you'd be better off in many cases not having one than a pubic pretender or the equivalent.
I agree that it is better to have a private attorney, but a public defender is better than nothing.