What is the deal on the "one drop rule"?

I hear this ALL the time. It's always black people who bring it up (I'm NOT saying all black people think this way, just the ones I've discussed it with). I mean 1) having a black ancestor from the 1700s or 1800s when the rule applied would not make you black (by this generation, you'd be less than 1% black genetically) and 2) Why would you promote something used to enslave and degrade people, including your ancestors? It's just stupid and has no basis.