Maximum drop protection requires maximum coverage from a shock absorbing material. Hard cases protect very little against shock damage, but are better than nothing. In reality though, most minimal coverage cases only protect against scratches.
Why would the "impact" go right to the glass of the iphone? Unless the case is touching the glass the "impact" can't go right to the glass. All the metal bumpers I have seen do not touch the glass directly and most have rubber or other material inlays on the inside of the case.
If you're looking for a serious drop protection a dumper is not the way to go metal, plastic or other.