Well what I think their logic is that since you can reorder a new passport within 6 months of an existing passport's expiration, that means you can potentially have 2 passports available, allowing the 'old' passport to be sold, maybe modified and used by the wrong people with still a valid expiration date.
SO I agree with most people here in that it's just not worth the hassles to not renew your passport. I did it last fall, it was easy to download the docs from online, fill them out and mail them in with my old passport with a USP money order. I got the new passport 3 weeks later....