Michael:

If many worlds meant infinitely many people this claim would be quite plausible to me, but why should aggregation stop mattering just because there are bignum people?

The point, I think, is not that there are bignum people, but that you don't have to worry about any possible people not 'getting the chance to live'. I see the appeal of this, but am not really swayed - Steven's last objection in particular seems very strong.