BX cable has a metal wound casing, and this casing has a high resistance over the length of the cable. If this casing is used as a grounding conductor, and a fault should occur, this casing will heat up and glow like a toaster element, resulting in a fire. Unlike MC cable, which has a grounding conductor running the full lenth of the cable and is rated for the fault current. I have personally wittnessed this on a service upgrade I was doing and about bit my scritches. So please, don't use the BX sheath for a grounding conductor unless you know what you are doing. One of the fixes in this instance that is approved by code, is to intall a GFCI rec.