Thorium is not a fissile isotope - meaning it can't be used for fission directly. It needs to be transmuted by neutrons into uranium 233 (which is fissile). By this measure, thorium isn't any better than Uranium 238 - which is 99.3% of naturally occurring uranium. So "All but a trace of the world's thorium exists as the useful isotope" - can be applied to uranium also. Uranium 238 is also a fertile isotope - and doing a completely fair comparison, uranium doesn't need to be enriched either... Except the only way we have to convert fertile isotopes to fissile isotopes is to expose them to a sustained, high neutron flux which is typically only economically achievable using enriched uranium (via the naturally occuring U235 isotope). Thorium breeder reactors need enriched uranium as much as uranium breeder reactors. So while thorium has some advantages, I wouldn't say that natural abundance or supply are particularly significant.

[Edit] corrected use of "fissile" -> "fertile"