The cover that you are missing is around $120 at the dealership, or much less at a wrecking yard. I have a '97 LS and plan on chaning mine out. Where it covers the oil filter has broken off and I would like to keep it in perfect shape.
IMO, I think it is a needed part for protection. To change the oil filter you have to remove 2 or 3 bolts and bend the cover out of the way.
BTW, I went and test drove a 01 LS430. Nice car, had bad tires, great power!