I rarely post here, but had to chime in after reading this. This is TERRIBLE advice. This is either flame or just pulled from the butt of a clueless/stupid 0L or 1L. If you want to become an ADA, DO NOT do work such as PD summer internships, death penalty clinics, etc.. Believe it or not, you'll be considered tainted and they won't want to touch you. They want the true believers only ;-)
As a 3L recently hired as an Assistant DA, this is absurd. Any decent DA's office is looking to hire good people with demonstrated commitment to public service -- people committed to doing justice and getting it right in every single case. The director of legal hiring at my office spent both summers in law school at a public defender's office, and many, many, many of recently hired ADAs have done public defense work and clinical work in law school.In fact, as someone who's resume reads "prosecutor, prosecutor, prosecutor," I was grilled about why I hadn't worked in indigent criminal defense. In many of the interview rounds they asked me if I was applying to public defenders offices, and if not, why? They are skeptical of people who say things like "I knew I wanted to be a prosecutor since I was 14..." In fact, 2 of the 5 elected DAs here (NYC) worked as public defenders before becoming ADAs or running for office. I will say that the reverse is not true, though. Many public defender's offices, for better or worse, will not consider someone who has a DA or USAO on their resume. That said, if you have any questions about DAs offices in NY and the surrounding areas, feel free to PM me. I was in your shoes 2 years ago, and I'd be happy to provide any inside info I have on internship opportunities here.