It was enabled but It was decided not to have because users leaving negative reputaion. If you think it has some positive outcomes let us know we will definitely think over enabling it again.
Reputation System is meant for both good and bad rep. If you are expecting good rep you should be able to expect bad rep also.
But the problem happens when people just gives out the bad ones and dont think of giving the good one.
There's always a limit...people can not give rep(bad or good) to same person again and again. I mean you should have rated some number of people before rating that same person again.
Its not about the same user giving the bad reputation but probably I wasnt clear and the case is if I like your post I should leave +ve rep and if not -ve but the natural tendency of the users is to forget about giving +ve and give away -ve fairly regularly without forgeting.