Definitely a nasty tax. Another example of why you should not locate your home or business in SF - the voters love to tax anything no matter what the harmful effects are.
I broadly share your view of San Francisco's civic culture. But what are the unexpected consequences of a vacancy tax? Practically every problem I think of, e.g. developers getting hit when the market goes down being forced to fire sell, has a simple solution (renting on short-term leases). The only failure mode is market breakdown, i.e. where homeowners trying to rent cannot find tenants at any price.