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.
Anyone need their essay read/edited? I may be able to critique some stuff for those interested. I've already submitted applications but you can still read my personal statement if you wish.
The fee waivers don't seem to be based on only stats alone. I've seen threads where people who have both lower LSAT scores and lower GPAs than me are getting fee waivers I didn't receive. I'm not sure entirely how it works. I did get Cornell's and not BU.