Suppose you can, as a package, bring seven lives into existence. Six will be good, one will be very bad. You might think it would be wrong to go ahead. Other things equal, starting good lives is permissible, but not starting bad lives is required. We can't justify starting this bad life by appeal to the good in other, separate, lives.
Predictably enough, I'm inclined to go 'indirect utilitarian' on this one, and grant it as a merely contingent moral principle. Context is all. As it happens, we have a high normal baseline: most lives in our society are pretty good. So, creating a good life is nothing exceptionally good, whereas creating a bad life is exceptionally bad. Given the more fundamental principle that exceptionally good or bad actions have greater moral significance (in virtue of their impact on the general form of society), we find that the contingent asymmetry in social circumstances leads to the above moral asymmetry.
But things could have been different. If we imagine a dystopian world where the normal 'baseline' is much lower, i.e. where most lives are rather awful, then it seems to me the moral asymmetry would be likewise inverted. Given the opportunity to bring about an exceptionally good life, people ought to do so. To prevent another typically bad one would be permissible -- good, even -- but not required. So, dystopians ought to embrace the 'package deal' of six good lives and one more bad one.
Benatar argues from the original asymmetry principle to the conclusion that no lives are worth living. (Even a mostly good life has some bad in it, so we are like the 6:1 package deal, squashed into one body.) But, ironically, his asymmetry principle only holds in the first place because of all the good lives that (future) people will have. In light of this contingency, Benatar's use of the principle is self-undermining. The prescribed "zero birthrate" would remove the grounds of its own justification.
In sum: the asymmetry principle derives whatever force it has from the more fundamental value of promoting the general welfare. So it cannot be used to subvert this very goal.