Adding a negative response button is like making free needles available to meth addicts. It will be abused and abused some more, until everyone is to tired to care much and their teeth fall out...
Seriously, the Internet is overwhelmingly a negative space where people are cloaked in anonymity. They tend to act out. Why facilitate that?
If I'd gotten into "dislike" wars with some of the complete morons who have posted here in the past trying to discredit their idiotic advice, I'd have spontaneously combusted long ago. It's not worth it. The "like" function is fine.