I plan to write them all. Like @"Cant Get Right" said above, they are "optional" but I think any serious candidate should write them. That's the general advice from admissions professionals, too.
It all depends on the school and the candidate. Certainly, a PS carries more weight for borderline candidates, but a bad PS (which includes a well-written statement with typos) can definitely ruin even a candidate who is above the medians for LSAT and GPA.
Are you considering Northwestern ED? Seems like you would be a really good candidate, especially if you're interested in Chi. and are debt averse. They'd appreciate all years of work experience more than most schools as well I'm thinking :)