If you can in good conscience say that you are committed to a career in the public interest, show the committee something tangible on your application and in your essay that will allow them to see your statements as more than mere assertions. If you cannot truthfully commit to a career spent Fighting the Good Fight, but you are willing to at least do your part, say so. Your essay might include a thoughtful discussion of your commitment to being a responsible, ethical professional while still being able to pay the rent. You should also realize, as the admissions committee will, that people often change their direction while in law school. The exact nature of the motivations you espouse may be less important than the way in which you discuss them.
Nothing is as impressive to the reader of a personal statement as the ring of truth.
For better or for worse, most law-school admissions committees group applications by grades and LSAT scores before getting to the essays. A school that does something like this will already know what they're looking for from your essay. That is to say that they already know what sort of consideration your grades and scores have earned you. The last thing the admissions committee needs to read is a laundry list of all your vital statistics. If you know that your grades and test scores are working against you, remember that the admissions committee knows this too.