Most states use rubber flashing instead of lead for the majority of their vent seals. Some mandate it because they dont want lead in their landfills. However, in the South, the sun is very hot and it rains a lot. Rubber tends to harden and eventually crack under intense sunlight, and these cracks lead to some serious leaks. So coastal South tends to use lead flashing rather than rubber. Now lead is used as far as Vancouver WA, but its not as extensive as rubber. Lead seals better then rubber, so the amount of rain does influence its use.
By covering with a Critterquitter, you protect it from squirrel damage if you live in a forest, so its worth the additional price to stop roof leaks.