I believe Eliezer's point is that the MWI nerfs the argument that we are depriving nonexistant people of their existence by not creating them. If the universe is as vast as the MWI implies then they already exist out there somewhere.

However, that particular argument isn't really a utilitarian argument in the first place. It's more of an egalitarian argument. I think a total utilitarian would be quite willing to never create a person if that person would be slightly less happy than the umpteenth copy of Felix.