Economically, creature X should exist if it wants to exist and it can pay for itself. That is, in a supply and demand world, if our only choice is whether X should exist, then an X that wants to exist should actually exist if its lifespan cost of resources used (including paying for any net externalities) is no more than the value it gives by working for others. [Similarly excerpted by Adam Ozimek on Modeled Behavior.]
In a comment on AO's post, RH says:
Surely there are many kinds of creatures where we could know with great confidence that they prefer to exist. Exact copies of other already existing creatures, for example. Can you accept that these creatures should exist?
I see a very serious problem with the move from "X creature is happy to exist" to "It is morally correct to make more creatures like X creature."
Say we make a golem out of clay, like in the old days. We bring it into existence to suffer a life of misery, as golems are want to have. But we endow it with a very special characteristic, along with life: the preference to exist. No matter what tortures we or the world inflict on our golem, it will keep on preferring to exist.
Is that moral? Can we create a Foxconn megafactory of such golems and keep them alive for miserable decade after miserable decade, with clean consciences?
The problem that I hope this raises is this: we expect preference to exist to be a function of quality of life, but it may actually be entirely independent from quality of life. People with every human advantage in the world (like me) often wish they had never been born; sick, suffering homeless people on the street often prefer to keep on living.
While I think we should respect an individual's decision as to whether it wishes to keep on living, this does not form a good guide as to whether to bring new people into existence.
The worst part: a pasted-on "preference for life" is exactly the sort of cruel trick we could expect evolution to play. What could be more beneficial? Except, perhaps, an unshakable preference to reproduce.