In my opinion most essays that concentrate on PI or saving the world sound very dochey, so I'd avoid that. Maybe, you should write about a case that you have worked for, how it challenged you, what emotions it aroused, show passion, but not directly.
for a personal statement on going into public interest law, how much experience in public interest would be needed to avoid sounding disingenuous? i have a year and a half of experience working fulltime at a public interest firm. should i still avoid the topic? would appreciate any input!
Yeah, avoid the topic. Adcomms know that you're probably going to change your mind as to what type of practice you'd like to do once you get there. You want to show the adcomms why they should choose you and to do that you've got to figure out why you're unique. Good luck.