wear-n-tear and mpgs were the original reasons Jeep put it in. then after a few years they removed it because it didn't help either thing plus added necessary cost and complexity.
the only real-world downside is that if your front driveshaft is worn out, it'll cause a vibration. but honestly, if it's worn out it should be addressed anyways. all XJs after the really early 90s, all ZJs, and all TJs were CAD free. plus IFS Jeeps like my liberty.