Since there are fewer and fewer VOR's, the options for alternative airports are limited if the weather goes bad. With RVAV's you can land at just about any airport.
VOR's approaches have higher minimum's than RNAV's, making it more likely to not be available. You might get lucky and find VOR navigation to an ILS approach, but those are fewer as well.
And given the state of repair with VOR's, and my personal experiences, the VOR you are depending on is much, much more likely to become inop than a GPS outage.