I find it useful for the minor diplomatic bonus if the civ in question isn't important. Like other people have stated, if there is a runaway civ, minmaxing involves making every decision that benefits you and hinders the opposing civ, they typically have a greater culture output due to size/population, so allowing for the open border tourism bonus is going to let them become influential with you faster. I'll usually only allow it for my vassals, as they have no chance of winning, and if you have vassals at all, everybody despises you for the sins it took to conquer.