Originally Posted by sweeks

This ^^. The chain-check tools serve as convenient "screening" tools... easy and fast to get a quick read on chain wear (and without getting fingers greasy). If excessive chain wear is indicated, then a "ruler" check can always be done. Since most chain-check tools seem to slightly over-estimate chain wear, the risk is that a chain with some remaining usable life will be discarded. I'd rather pitch a chain that I *could* get a little more life out of, than wear out my cassette or chainrings.