Because 2 1/2 years ago I spent $300 on a non-stainless cat-back exhaust from A-B. Now it is all rusted-out & the muffler rattles terribly. I don't feel like spending $300 & a couple hours of my time every 2 1/2 years to replace the exhaust. And one of my cats has also failed, so I could use a manifold-back exhaust. I have a parts DII sitting in the shed with a perfectly fine factory stainless exhaust. Makes sense to me to put it to good use.
After I get my head gasket job done on my '03, and it is back on the road, I'll try the exhaust swap.